THE    WILEY    TECHNICAL    SERIES 

FOR 

VOCATIONAL    AND    INDUSTRIAL    SCHOOLS 

EDITED   BY 

JOSEPH    M.    JAMESON     ., 

'*•        GIRARD  COLLEGE 


THE  WILEY  TECHNICAL  SERIES 

EDITED    BY 

JOSEPH  M.  JAMESON 


TEXT  BOOKS  IN 
MECHANICS,  HEAT  AND  POWER 

NOW   READY 

Steam  Power.  By  C.  F.  HIBSHFELD.  Professor  of 
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Gas  Power.  By  C.  F.  HIRSHFELD,  Professor  of  Power 
Engineering,  Sibley  College,  Cornell  University, 
and  T.  C.  ULBRICHT,  Formerly  Instructor,  Depart- 
ment of  Power  Engineering,  Cornell  University. 
x+209  pages,  5J  by  8,  60  figures.  Cloth,  $1.25 
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5000  8-1-16 


STEAM  POWER 


BY 

C.  F.  HIRSHFELD,  M.M.E. 

PROFESSOR   OF  POWER  ENGINEERING,   SIBLEY   COLLEGE,    CORNELL  UNIVERSITY 

AND 

T.  C.  ULBRICHT,  M.E.,  M.M.E. 

FORMERLY  INSTRUCTOR,  DEPARTMENT  OF  POWER  ENGINEERING,  SIBLBY  COLLEGE, 

CORNELL    UNIVERSITY;    ASSOCIATE    MEMBER    AMERICAN 
SOCIETY  OF  MECHANICAL  ENGINEERS 


FIRST    EDITION 

FIRST   THOUSAND 


NEW  YORK 

JOHN   WILEY  &   SONS,   INC. 

LONDON:   CHAPMAN  &    HALL,   LIMITED 

1916 


•f 


COPYRIGHT,  1916,  BT 
C.  F.  HIRSHFELD  AND  T.  C.  ULBRICHT 


PRESS     OF 

BRAUNWORTH    &    CO. 

BOOKBINDERS    AND    PRINTERS 

BROOKLYN,    N.    Y- 


PREFACE 


THE  following  pages  represent  the  results  of  an  attempt 
to  collect  in  a  comparatively  small  book  such  parts  of  the 
field  of  steam  power  as  should  be  familiar  to  engineers 
whose  work  does  not  require  that  they  be  conversant  with 
the  more  complicated  thermodynamic  principles  considered 
in  advanced  treatments.  The  experience  of  the  authors 
has  led  them  to  believe  that  a  book  of  this  sort  should  give 
a  correct  view-point  with  regard  to  the  use  of  heat  in  the 
power  plant  even  though  it  does  not  enter  deeply  into  the 
theoretical  considerations  leading  up  to  that  view-point; 
that  it  should  supply  the  tools  required  for  the  solution  of 
power  plant  problems  of  the  common  sort ;  and  that  it  should 
give  sufficient  description  of  power  plant  apparatus  to 
make  the  reader  fairly  familiar  with  the  more  common 
types. 

Mathematical  treatment  of  the  subject  has  been  elim- 
inated to  the  greatest  possible  extent,  and  anyone  familiar 
with  elementary  algebra  should  be  able  to  understand 
readily  such  equations  as  it  has  been  deemed  necessary  to 
include. 

Brief  explanations  of  physical  and  chemical  concepts 
are  given  in  every  case  in  which  the  text  required  their  use, 
so  that  those  who  have  not  studied  these  subjects,  and  those 
who  have  but  have  failed  to  crystallize  and  hold  the  neces- 

iii 

342913 


iv  PEEFACE 

sary  ideas,  should  have  little  difficulty  in  reading  the  text 
understandingly. 

It  is  hoped  that  the  book  may  prove  serviceable  as  a 
text  for  steam  power  courses  given  to  civil  engineers  in  the 
various  colleges  and  that  it  may  also  meet  the  needs  of  those 
instructing  power  plant  operators  in  industrial  schools. 

C.  F.  H. 

T.  C.  U. 

JUNE,  1916. 


CONTENTS 


CHAPTER  I 

PAGE 

PHYSICAL  CONCEPTIONS  AND  UNITS 1 

1.  Matter.  2.  Energy.  3.  Units  of  Matter  and  Energy. 
4.  Work.  5.  Mechanical  Energy.  6.  Heat.  7.  Temper- 
ature. 8.  Measurement  of  Temperature.  9.  The  Unit  of 
Heat  Energy.  10.  Specific  Heat.  11.  Quantity  of  Heat. 
12.  Work  and  Power. 

CHAPTER  II 

THE  HEAT-POWER  PLANT 20 

13.  The  Simple  Steam-power  Plant.  14.  Cycle  of  Events. 
15.  Action  of  Steam  in  the  Cylinder.  16.  Hydraulic  Analogy. 

CHAPTER  III 

STEAM 27 

17.  Vapors  and  Gases.  18.  Properties  of  Steam.  19. 
'Generation  of  Steam  or  Water  Vapor.  20.  Heat  of  Liquid, 
q  or  h.  21.  Latent  Heat  of  Vaporization,  r  or  L.  22.  Total 
Heat  of  Dry  Saturated  Steam,  X  or  H.  23.  Total  Heat  of  Wet 
Steam.  24.  Heat  of  Superheat.  25.  Total  Heat  of  Super- 
heated Steam.  26.  Specific  Volume  of  Dry  Saturated 
Steam,  V  or  S.  27.  Specific  Density  of  Dry  Saturated 

Steam,  —  or  6.  28.  Reversal  of  the  Phenomena  Just  De- 
scribed. 29.  Generation  of  Steam  in  Real  Steam  Boiler. 
30.  Gauge  Pressure. 

CHAPTER  IV 

THE  IDEAL  STEAM  ENGINE 43 

31.  The  Engine.  32.  Operation  of  the  Engine.  33.  Work 
Done  by  the  Engine.  34.  Heat  Quantities  Involved.  35. 
Efficiency.  36.  Effect  of  Wet  Steam.  37.  Application  to 


vi  CONTENTS 

PAGE 

a  Real  Engine.  38.  Desirability  of  Other  Cycles.  39.  The 
Complete  Expansion  Cycle.  40.  The  Incomplete  Expan- 
sion Cycle. 

CHAPTER  V 

ENTROPY  DIAGRAM 61 

41.  Definitions.  42.  Temperature-Entropy  Chart  for 
Steam.  43.  Quality  from  TV-Chart.  44.  Volume  from  T<£- 
chart.  45.  Heat  from  T<£-chart.  46.  The  Complete  T0- 
chart  for  Steam. 

CHAPTER  VI 

TEMPERATURE  ENTROPY  DIAGRAMS  OF  STEAM  CYCLES 72 

47.  Complete-expansion  Cycle.  48.  Area  of  Cycle  Repre- 
sentative of  Work.  49.  Modifications  for  Wet  and  Super- 
heated Steam.  50.  Incomplete  Expansion  Cycle.  51. 
Effect  of  Temperature  Range  on  Efficiency. 

CHAPTER  VII 

THE  REAL  STEAM  ENGINE » 77 

52.  Operations  of  Real  Engine.  53.  Losses  in  Real  In- 
stallations. 54.  Clearance.  55.  Cushion  Steam  and  Cylinder 
Feed.  56.  Determination  of  Initial  Condensation.  57. 
Methods  of  Decreasing  Cylinder  Condensation.  58.  Classi- 
fication of  Steam  Engines.  59.  Rotative  Speeds  and  Piston 
Speeds.  60.  The  Simple  D-Slide  Valve  Engine.  61.  Engine 
Nomenclature.  62.  Principal  Parts  of  Engines. 

CHAPTER  VIII 

THE  INDICATOR  DIAGRAM  AND  DERIVED  VALUES 115 

63.  The  Indicator.  64.  Determination  of  I.h.p.  65. 
Conventional  Diagram  and  Card  Factors.  66.  Ratio  of 
Expansion.  67.  Determination  of  Clearance  Volume  from 
Diagram.  68.  Diagram  Water  Rate.  69.  TV-diagram  for 
a  Real  Engine.  70.  Mechanical  and  Thermal  Efficiencies. 

CHAPTER  IX 

COMPOUNDING 141 

71.  Gain  by  Expansion.  72.  Compounding.  73.  The 
Compound  Engine.  74.  Cylinder  Ratios.  75.  Indicator 
Diagrams  and  Mean  Pressures.  76.  Combined  Indicator 
Diagrams. 


CONTENTS  vii 

CHAPTER  X 

PAGE 

THE  D-SLIDE  VALVE 159 

77.  Description  and  Method  of  Operation.  78.  Steam  Lap. 
79.  Lead.  80.  Angle  of  Advance.  81.  Exhaust  Lap.  82. 
The  Bilgram  Diagram.  83.  Exhaust  and  Compression.  84. 
Diagram  for  Both  Cylinder  Ends.  85.  Piston  Positions. 
86.  Indicator  Diagram  from  Bilgram  Diagram.  87.  Limita- 
tions of  the  D-slide  Valve.  88.  Reversing  Engines.  89,  Valve 
Setting. 

CHAPTER  XI 

CORLISS  AND  OTHER  HIGH-EFFICIENCY  ENGINES 196 

90.  The  Trip-cut-off  Corliss  Engine.  91.  Non-detaching 
Corliss  Gears.  92.  Poppet  Valves.  .  93.  The  Una-flow  En- 
gine. 94.  The  Locomobile  Type. 

CHAPTER  XII 

REGULATION 213 

95.  Kinds  of  Regulation.  96.  Governor  Regulation.  97. 
Methods  of  Varying  Mean  Effective  Pressure.  98.  Con- 
stant Speed  Governing.  99.  Governors. 

CHAPTER  XIII 

THE  STEAM  TURBINE 221 

100.  The  Impulse  Turbine.  101.  Theoretical  Cycle  of 
Steam  Turbine.  102.  Nozzle  Design.  103.  Action  of  Steam 
on  Impulse  Blades.  104.  De  Laval  Impulse  Turbine.  105. 
Gearing  and  Staging.  106.  The  Reaction  Type.  107.  Com- 
bined Types.  108.  Economy  of  Steam  Turbines. 

CHAPTER  XIV 

CONDENSERS  AND  RELATED  APPARATUS 251 

109.  The  Advantage  of  Condensers.  110.  Measurement 
of  Vacuums.  111.  Conversion  of  Readings  from  Inches  of 
Mercury  to  Pounds  per  Square  Inch.  112.  Principle  of  the 
Condenser.  113.  Types  of  Condensers.  114.  The  Jet  Con- 
denser. 115.  Non-Contact  Condensers.  116.  Water  Re- 
quired by  Contact  Condensers.  117.  Weight  of  Water 
Required  by  Non-contact  Condensers.  118.  Relative  Ad- 
vantages of  Contact  and  Surface  Condensers.  119.  Cool- 
ing Towers. 


viii  CONTENTS 

CHAPTER  XV 

PAGE 

COMBUSTION 277 

120.  Definitions.  121.  Combustion  of  Carbon.  122. 
Combustion  to  CO.  123.  Combustion  to  CO2.  124.  Com- 
bustion of  CO  to  CO2.  125.  Conditions  Determining  Forma- 
tion of  CO  and  CO2.  126.  Flue  Gases  from.  Combustion  of 
Carbon.  127.  Combustion  of  Hydrogen.  128.  Combustion 
of  Hydrocarbons.  129.  Combustion  of  Sulphur.  130.  Com- 
bustion of  Mixtures.  131.  Temperature  of  Combustion. 

CHAPTER  XVI 

FUELS 296 

132.  Commercial  Fuels.  133.  Coal.  134.  Coal  Analyses. 
135.  Calorific  Value  of  Coals.  136.  Purchase  of  Coal  on 
Analysis.  137.  Petroleum. 

CHAPTER  XVII 

STEAM  BOILERS 305 

138.  Definitions  and  Classifications.  139.  Functions  of 
Parts.  140.  Furnaces  and  Combustion.  141.  Hand  Firing. 
142.  Mechanical  Grates.  143.  Smoke  and  its  Prevention. 
144.  Mechanical  Stokers.  145.  Rate  of  Combustion.  146. 
Strength  and  Safety  of  Boiler.  147.  Circulation  in  Boilers. 
148.  Types  of  Boilers.  149.  Boiler  Rating.  150.  Boiler  Effi- 
ciencies. 151.  Effects  of  Soot  and  Scale.  152.  Scale.  153. 
Scale  Prevention.  154.  Superheaters.  155.  Draft  Apparatus. 

CHAPTER  XVIII 

RECOVERY  OF  WASTE  HEAT 375 

156.  Waste  Heat  in  Steam  Plant.  157.  Utilization  of 
Exhaust  for  Heating  Buildings.  158.  Feed-water  Heating. 

CHAPTER  XIX 

BOILER-FEED  PUMPS  AND  OTHER  AUXILIARIES 382 

159.  Boiler-feed  Pumps.  160.  The  Steam  Injector.  161. 
Separators.  162.  Steam  Traps.  163.  Steam  Piping. 


STEAM    POWER 


CHAPTER   I 
PHYSICAL   CONCEPTIONS  AND   UNITS 

1.  Matter.  The  universe  is  generally  pictured  as  com- 
posed of  matter  and  energy.  Matter  is  regarded  as  that 
which  is  possessed  of  mass,  or  as  that  which  is  possessed  of 
inertia;  i.e.,  which  requires  the  action  of  force  to  put  it  in 
motion,  to  bring  it  to  rest  or  to  change  its  velocity.  These 
definitions  merely  enumerate  characteristics  of  matter;  they 
do  not  tell  what  it  really  is.  In  the  present  state  of  knowledge 
it  is,  however,  impossible  to  define  matter  in  any  other  way. 

No  experiment  has  yet  shown  that  matter  can  be  created 
or  destroyed  by  man.  It  can  be  changed  from  one  form  to 
another,  it  can  be  given  certain  physical  and  certain  chem- 
ical characteristics,  more  or  less  at  will,  but  the  actual 
quantity  of  matter  concerned  is  always  the  same  after  and 
before  such  changes.  It  is  customary  to  state  this  experi- 
ence in  the  form  of  a  law  known  as  the  Law  of  the  Con- 
servation of  Matter,  which  states  that  the  "  total  quantity 
of  matter  in  the  universe  is  constant." 

Matter  is  known  to  exist  in  several  physical  states  or 
conditions  of  aggregation.  The  three  most  familiar  are  (1) 
solid,  (2)  liquid  and  (3)  gaseous.  In  each  of  these  states 
matter  is  conceived  as  made  up  of  minute  particles  called 
molecules  which  in  turn  are  apparently  composed  of  still 
smaller  parts  known  as  atoms.  These  atoms  can  also  be 
broken  into  parts,  but  for  the  purposes  of  this  book  it  is  not 
necessary  to  consider  such  divisions. 


2  STEAM  POWER 

Experiment  and  mathematical  reasoning  seem  to  indi- 
cate that  the  molecules  of  all  materials  are  in  constant 
motion  and  that  there  are  neutralizing  attractive  and  repul- 
sive forces  acting  between  them.  In  solids  the  molecules  are 
apparently  bound  together  in  such  a  way  that,  although  they 
are  in  constant  motion,  the  external  form  or  shape  of  the 
body  tends  to  remain  constant;  in  fact  it  requires  the 
expenditure  of  force  to  cause  a  change  of  form.  In  liquids 
the  molecular  attraction  is  so  altered  that  practically  all 
rigidity  disappears  and  the  shape  assumed  by  the  liquid  is 
determined  by  that  of-  the  surrounding  surfaces,  as,  for 
instance,  the  shape  of  the  vessel  containing  the  liquid.  In 
gases  the  molecules  are  still  more  free  and  actually  tend  to 
move  apart  as  far  as  possible,  so  that  a  gas  will  spread  in 
all  directions  until  it  fills  any  closed  containing  vessel. 

2.  Energy.  Nearly  everyone  has  a  conception  of  what 
is  meant  by  the  term  energy,  but  no  one  yet  knows  what 
energy  really  is.  It  is  defined  as  the  capacity  for  doing  work, 
or  the  ability  to  overcome  resistance.  A  man  is  said  to  be  very 
energetic  or  to  be  possessed  of  a  great  deal  of  energy  when 
he  has  the  ability  to  perform  a  great  amount  of  work  or 
to  overcome  great  resistances.  Matter  is  said  to  be  pos- 
sessed of  energy  when  it  can  perform  work  or  overcome 
resistance.  Actually,  matter  is  not  known  in  any  form  in 
which  it  is  not  possessed  of  energy. 

There  are  many  different  forms  of  energy.  A  body  in 
motion  can  do  work  and  is  said  to  be  possessed  of  mechani- 
cal energy.  A  body  which  we  recognize  as  hot  can  do  work 
at  the  expense  of  the  heat  associated  with  it  and  is  said 
to  be  possessed  of  heat  energy.  Light,  sound  and  electricity 
are  all  forms  of  energy. 

Experiment  and  experience  have  never  shown  that  energy 
can  be  destroyed  or  created  by  man,  but  they  have  shown 
that  one  form  of  energy  can  be  converted  into  another  form 
under  proper  conditions.  The  first  part  of  this  experience 
is  stated  as  a  law  known  as  the  Law  of  the  Conservation  of 


PHYSICAL  CONCEPTIONS  AND  UNITS 


Energy.     This  law  states  that  "  the  total  quantity  of  energy 
in  the  universe  is  constant." 

3.  Units  of  Matter  and  of  Energy.     When  attempts  are 
made  to  measure  the  amount  of  anything,  some  unit  of 
measurement  is  adopted.     Matter  is  measured  in  numerous 
ways  and  many  units  are  used.     The  common  methods  of 
measuring  matter  are  by  volume  and  by  weight.     Engineers 
in  English-speaking  countries  use  the  cubic, yard,  the  cubic 
foot  or  the  cubic  inch  as  units  in  measuring  matter  by  vol- 
ume and  they  use  the  pound,  the  ounce,  the  grain,  etc.  as 
units  in  measuring  matter  by  weight. 

Energy  is  measured  in  many  units  and,  in  general,  there 
is  a  characteristic  unit  or  set  of  units  for  each  form  in  which 
it  occurs.  Thus  the  foot-pound  is  very  commonly  used  for 
measuring  mechanical  energy;  the  British  thermal  unit  for 
measuring  heat  energy;  and  the  joule  for  measuring  electrical 
energy.  Some  of  these  units  will  be  defined  and  considered 
in  greater  detail  in  subsequent  paragraphs. 

4.  Work.   Work  is  defined  as  the  overcoming  of 
a  resistance  through  a  distance.    Thus,  work  is  done 
when  a  weight  is    raised  against  the  resistance 
offered  by  gravity;  work  is  done  when  a  spring  is 
compressed    against    the    resistance    which    the 
metal  offers  to  change  of  shape;   work  is  done 
when  a  body  is  moved  over  another  against  the 
resistance  offered  by  friction. 

The  unit  of  work  is  the  quantity  of  work  which 
must  be  done  in  raising  a  weight  of  one  pound 
through  a  vertical  distance  of  one  foot.  It  is  called 
the  foot-pound.  Thus,  one  foot-pound  of  work 
must  be  done  in  raising  one  pound  one  foot;  two 
foot-pounds  of  work  must  be  done  in  raising  two 
pounds  one  foot  or  in  raising  one  pound  two  feet. 

If  a  weight  of  one  pound  were  suspended  from 
a  spring  balance  as  shown  in  Fig.  1,  the  balance  would  in- 
dicate a  pull  or  force  of  one  pound.     No  work  would  be 


FIG.  1. 


4  STEAM  POWER 

done  by  this  force  as  long  as  the  weight  remained  stationary, 
because  no  resistance  would  be  overcome  through  a  distance. 
If,  however,  the  same  weight  were  slowly  or  rapidly  raised 
a  vertical  distance  of  a  foot,  one  foot-pound  of  work  would 
be  done.  A  force  or  pull  of  one  pound  would  then  have 
overcome  a  resistance  of  one  pound  through  a  distance  of 
one  foot.  In  general: 

Work  in  ft. -Ibs.  =  Resistance  overcome  in  Ibs.X  distance. 
=  Force  in  Ibs.X  distance  in  ft. 

so  that  if  a  force  of  10  Ibs.  pushes  or  pulls  anything  which 
offers  a  resistance  of  10  Ibs.  while  that  something  travels 
a  distance  of,  say,  5  ft.,  the  work  done  will  be  given  by  the 
expression, 

Work  =  10X5, 

=  50ft.-lbs. 

A  body  in  falling  a  certain  distance  can  do  work  equal  to 
its  weight  multiplied  by  the  distance  it  falls  because  it  could 
theoretically  raise  an  equal  weight  an  equal  distance  against 
the  action  of  gravity,  and  the  work  done  upon  this  second 
body  would  be  equal  to  its  weight  multiplied  by  the  distance 
through  which  it  was  raised. 

It  is  very  important  to  note  that  no  work  is  done  by  a 
force  if  there  is  no  motion;  resistance  must  be  overcome 
through  a  distance  in  order  that  work  may  be  done.  Thus, 
a  force  of  1000  Ibs.  might  be  required  to  hold  something  in 
position,  that  is  to  balance  a  resistance,  but  no  work  wrould 
be  done  if  the  body  upon  which  the  1000-pound  force  acted 
did  not  move.  Again,  a  weight  of  50  Ibs.  held  at  a  distance  of 
10  ft.  above  the  surface  of  the  earth  would  exert  a  downward 
push  or  pull  equal  to  50  Ibs.  on  whatever  held  it  in  that 
position;  it  would,  however,  do  no  work  if  held  in  that 
position.  If  allowed  to  fall  through  the  distance  of  10  ft. 
it  could  do  50X10  =  500  ft.-lbs.  of  work. 

It  is  very  convenient  to  represent  graphically  the  action 


PHYSICAL  CONCEPTIONS  AND  UNITS  5 

of  forces  overcoming  resistances,  that  is,  doing  work.  This 
is  done  by  plotting  points  showing  the  magnitude  of  the 
force  at  the  time  that  the  body  on  which  it  is  acting  has 
traveled  different  distances.  Thus,  suppose  a  constant 
force  of  10  Ibs.  pushes  a  body  a  distance  of  15  ft.  against  a 
constant  resistance  of  10  Ibs.  The  force  acting  on  the  body 
will  have  a  value  of  10  Ibs.  just  as  the  body  starts  to  move, 
a  value  of  10  Ibs.  when  the  body  has  moved  1  ft.,  a  value 
of  10  Ibs.  when  the  body  has  moved  2  ft.,  and  so  on.  This 
might  be  represented  by  points  on  squared  paper  as  shown 


\z 

11 

Force  in  Pounds  . 

H-*  tC  Oi  *"  C<  05  ->  ;x  CO  C 

0        1        2        3       4        56        7        8        9       10      11      12      13      14      15 

Distance  traveled  in  Feet 


FIG.  2. 


in  Fig.  2  or  by  a  horizontal  line  joining  those  points  as  shown 
in  the  same  figure. 

The  work  done  by  this  force  would  be  10X15  =  150 
ft.-lbs.  according  to  our  previous  definition.  But  10X15 
is  also  the  number  of  small  squares  under  the  line  represent- 
ing the  action  of  this  force  in  Fig.  2.  The  number  of  these 
small  squares  then  must  be  a  measure  of  the  work  done, 
but  it  is  also  a  measure  of  the  area  under  the  line  represent- 
ing the  action  of  the  force,  so  that  this  area  must  be  a  measure 
of  the  work  done.  Each  small  square  represents  1  Ib.  by 
its  vertical  dimension  and  l.ft.  by  its  horizontal  dimension, 


6 


STEAM  POWER 


so  that  its  area  must  represent  1  Ib.Xl  ft.  =  1  ft.-lb. 
The  total  number  of  squares  below  the  line  equals  10  X 
15  =  150,  and  since  the  area  of  each  one  represents  1  ft.-lb. 
the  total  area  under  the  line  represents  150X1  =  150 
ft.-lbs. 

It  is  not  always  convenient  to  choose  such  simple  scales 
as  those  just  used.  Thus  it  might  be  more  convenient 
to  plot  the  action  of  this  force  as  is  done  in  Fig.  3.  Here 
the  height  of  a  square  represents  2  Ibs.  and  the  width 
represents  1  ft.;  the  area  then  represents  2X1  =  2  ft.-lbs. 
There  are  5X15  =  75  squares  under  the  line  and  as  each 


.12 


567         8        9       10      11 
Distance  traveled  in  Feet 

FIG.  3. 


12      13      14      15 


represents  2  ft.-lbs.  the  total  area  under  the  line  represents 
2X75  =  150  ft.-lbs.  as  before. 

This  is  a  very  useful  property  of  these  diagrams  and  the 
area  under  the  line  representing  the  action  of  the  force 
always  represents  the  work  done,  no  matter  what  the  shape 
of  that  line. 

Thus,  assume  a  force  which  compresses  a  spring  a  distance 
of  6  ins.  Suppose  that  a  force  of  10  Ibs.  is  required  to  com- 
press the  spring  1  in.,  a  force  of  20  Ibs.  to  compress  it  2  ins., 
and  so  on  up  to  a  force  of  60  Ibs.  to  compress  it  6  ins. 
Starting  with  a  force  of  zero,  the  force  will  have  to  gradually 
increase  as  the  spring  is  compressed,  as  shown  by  the  line  in 
Fig.  4.  The  area  of  each  of  the  small  squares  will  represent 

=—  ft.-lbs.     Under  the  line  there  is  an  area  equal 

1-     '  - 


PHYSICAL  CONCEPTIONS  AND  UNITS 


/>  \/  £* 

o  -  —=18  small  squares,  and  the  work  done  in  compressing 
A 

the  spring  must  then  be  18X^  =  15  ft.-lbs. 

5.  Mechanical  Energy.  Any  body  which  exists  in  such 
a  position  or  location  that  it  could  do  work  by  dropping  or 
falling  is  said  to  be  possessed  of  potential  mechanical  energy, 
or  of  mechanical  energy  due  to  position.  As  long  as  it 
remains  in  this  position,  it  cannot  do  work  at  the  expense 
of  this  energy,  but,  if  allowed  to  fall,  it  could  do  so.  The 


8 


e  in  Pounds 

g  S 


I 


1"         2"        3"        4"         5"        6" 
K2  Ft.  %  Ft.  %z  Ft. 

FIG.  4. — Graph  Showing  Action  of  Spring. 

work  it  could  do  would  be  equal  to  the  product  of  its  weight 
by  the  distance  it  could  fall  and  the  potential  energy  it 
possesses  before  starting  to  fall  is  measured  by  this  work. 
Thus,  a  body  weighing  40  Ibs.  located  10  ft.  above  the  surface 
of  the  earth  could  do  40X10  =  400  ft.-lbs.  of  work  in  falling, 
and,  therefore,  it  is  said  to  be  possessed  of  400  ft.-lbs.  of 
potential  energy  before  it  starts  to  fall. 

If  in  falling  it  raises  a  weight  equal  to  its  own  (theo- 
retically) through  a  distance  equal  to  that  through  which 
it  falls  (theoretically),  it  will  have  used  up  400  ft.-lbs.  of 
energy  in  doing  400  ft.-lbs.  of  work  upon  the  body  raised 


8  STEAM  POWER 

and  will  no  longer  be  possessed  of  that  amount  of  potential 
energy.  The  body  which  has  been  raised  will,  however, 
have  an  equal  amount  of  energy  stored  in  it  and  will  in  turn 
be  able  to  do  400  ft.-lbs.  of  work  if  allowed  to  fall  a  distance 
of  10  ft. 

If  the  body  assumed  above  falls  through  a  distance  of 
10  ft.  without  raising  another  body  or  doing  an  equivalent 
amount  of  work  in  some  other  way,  it  acquires  a  high 
velocity.  When  it  arrives  at  the  bottom  of  the  fall  of  10 
ft.,  it  certainly  does  not  possess  the  400  ft.-lbs.  of  potential 
energy  which  it  had  before  dropping  nor  has  it  done  work 
at  the  expense  of  that  energy.  Moreover,  the  energy  could 
not  have  been  destroyed  because  it  is  indestructible.  The 
only  conclusion  is  that  it  must  still  be  possessed  of  this 
energy  in  some  way.  At  the  end  of  the  fall  it  has  lost  its 
advantageous  position,  but  it  has  acquired  a  high  velocity, 
and  experience  shows  that  if  brought  to  rest  it  can  do 
work  upon  that  which  brings  it  to  rest  equal  to  what 
it  could  have  done  in  raising  a  weight  as  previously 
described. 

At  the  end  of  its  fall  and  before  being  brought  to  rest, 
the  body  is  therefore  said  to  be  possessed  of  energy  by  virtue 
of  its  velocity,  and  this  form  of  energy  is  called  kinetic 
mechanical  energy.  The  kinetic  energy  will  be  exactly  equal 
to  the  potential  energy  which  disappeared  as  the  body  fell. 

Any  body  which  is  moving  is  possessed  of  kinetic  energy 
because  it  can  do  work  on  anything  which  brings  it  to 
rest.  This  energy  is  expressed  by  the  equation, 

1       W 

Kinetic  Energy  in  ft.-lbs.=-Xi^^X  V2, 

_.     .  >_ ._ 

in  which 

'  W  =  the  weight  of  the  moving  body  in  pounds. 

F  =  the  velocity  in  ft.  per  second,  and 
32.2  =  a  gravitational  constant  commonly  represented  by  g 


PHYSICAL  CONCEPTIONS  AND  UNITS  9 

6.  Heat.  One  of  the  most  familiar  forms  of  energy  is 
heat,  which  manifests  itself  to  man  through  the  sense  of 
touch.     In  reality  every  body  with  which  man  is  familiar 
possesses  an  unknown  amount  of  heat  energy   and  it  is 
assumed  that  this  heat  energy  is  in  some  way  associated 
with  the  motions  and  relative  positions  of  the  molecules  and 
their  constituents. 

For  this  reason  heat  is  often  described  as  molecular 
.activity  and  is  regarded  as  energy  stored  up  in  a  substance  by 
virtue  of  its  molecular  condition.  Heat  energy  can  be  made 
to  perform  work  in  ways  which  will  be  discussed  later  and 
this  is  proof  that  it  is  a  form  of  energy  and  not  a  material 
substance,  as  was  once  supposed. 

Heat  is  observed  and  recorded  by  its  effects  on  matter, 
producing  changes  in  the  dimensions  or  volumes  of  objects; 
changes  of  internal  stress;  changes  of  state,  as  ice  to  water 
and  water  to  steam;  changes  of  temperature;  and  electrical 
and  chemical  effects. 

Neglecting  certain  atomic  phenomena  not  yet  well  under- 
stood, the  probable  source  of  all  heat  energy  appearing  on 
the  earth  is  the  sun.  Heat,  however,  may  be  obtained 
from  mechanical  and  electrical  energy;  from  chemical 
changes;  from  changes  of  physical  state;  from  the  internal 
heat  of  the  earth. 

7.  Temperature.     Man  early  realized  that  under  certain 
conditions  bodies  felt  "  hotter  "  than  under  other  conditions 
and  gradually  came  to  speak  of  the  "  degree  of  hotness  "  as 
the  temperature  of  the  body.     It  was  later  realized  that  what 
was  really  measured  as  the  "  hotness  "  or  intensity  of  heat 
or  temperature  of  a  body  was  the  ability  of  that  body  to  trans- 
mit heat  to  others  and  that  it  had  no  connection  with  quantity 
of  heat. 

Thus,  if  the  temperature  of  two  adjacent  bodies  happened 
to  be  the  same,  one  of  them  could  not  lose  heat  by  trans- 
mitting it  to  the  other,  but  if  the  temperature  of  one 
happened  to  be  higher  than  that  of  another,  the  body  at 


10  STEAM  POWER 

higher  temperature  would  always  lose  heat  to  the  one  at 
lower  temperature. 

As  a  means  of  measuring  temperature  certain  arbitrary 
scales  have  been  chosen.  The  centigrade  scale  of  tempera- 
ture, for  instance,  is  based  upon  the  temperatures  of  melting 
ice  and  boiling  water  under  atmospheric  pressure.  The  tem- 
perature difference  between  boiling  .water  at  atmospheric 
pressure  and  melting  ice  at  atmospheric  pressure  is  arbi- 
trarily called  one  hundred  degrees  of  temperature,  and  the 
temperature  of  the  melting  ice  is  called  zero,  making  that 
of  the  boiling  water  100  degrees. 

Any  body  which  has  such  a  temperature  that  it  will  not 
give  heat  to,  or  take  heat  from,  melting  ice  is  said  to  be  at 
a  temperature  of  zero  degrees  centigrade,  represented  as  0°  C. 
Similarly,  any  body  in  such  a  condition  that  it  will  not  give 
heat  to  or  take  heat  from  water  boiling  under  atmospheric 
pressure  is  said  to  have  a  temperature  of  100°  C.  A  body 
with  a  temperature  exactly  half  way  between  these  two 
limits  would  then  be  said  to  have  a  temperature  of  50°  C. 

8.  Measurement  of  Temperature.  The  temperatures  of 
bodies  could  be  determined  by  bringing  them  in  contact  with 
such  things  as  melting  ice  and  boiling  water  and  determining 
whether  or  not  a  transfer  of  heat  occurred,  but  this  would 
be  a  very  cumbersome  and  unsatisfactory  method.  As  a 
consequence  many  other  means  have  been  devised  for  the 
measurement  of  temperature. 

One  of  the  most  common  and  convenient  methods  in- 
volves the  use  of  what  are  known  as  mercury  thermometers. 
These  depend  upon  the  fact  that  the  expansion  of  mercury 
with  changing  temperature  is  very  uniform  over  a  wide 
temperature  range.  Thus,  if  mercury  expands  a  certain 
amount  when  its  temperature  is  raised  from  that  of  melting 
ice  to  that  of  boiling  water,  i.e.,  100°  C.,  it  will  expand  just 
half  as  much  when  its  temperature  is  raised  half  as  high, 
and  one-quarter  as  much  when  its  temperature  is  raised  one- 
quarter  of  the  range  from  0°  to  100°  C. 


PHYSICAL  CONCEPTIONS  AND  UNITS 


11 


The  thermometer  is  made  by  enclosing  a  small  quan- 
tity of  mercury  in  a  glass  tube  fitted  with  a  bulb  at 
one  end,  as  shown  in  Fig.  5.  The  lower  end 
of  the  thermometer  is  immersed  in  melting 
ice  and  the  point  on  the  stem  which  is 
reached  by  the  top  of  the  mercury  column  is 
marked  and  labelled  0°  C.  The  thermometer 
is  then  immersed  in  the  steam  from  water  boil- 
ing under  atmospheric  pressure  and  the  point 
reached  by  the  top  of  the  mercury  column  is 
marked  and  labelled  100°  C.  The  distance  be- 
tween the  two  marks  is  then  divided  into  one 
hundred  parts  and  each  represents  the  distance 
which  the  end  of  the  column  of  mercury  will 
move  when  its  temperature  changes  one  centi- 
grade degree. 

It  is  customary  to  extend  this  same  scale 
below  0°  and  above  100°,  carrying  it,  on  ex- 
pensive thermometers,  as  far  in  each  direction 

, !  .        ,  •  •       FIG.  5.— Mer- 

as  the  approximation  to  a  constant  expansion     cur    Ther 

on  the  part   of  the  mercury  and   to   constant     m0meter. 
properties  of  the  glass  justifies. 

The  temperature  of  a  body  can  then 
be  found  by  placing  the  thermometer  in 
or  in  contact  with  that  body  and  noting 
the  point  reached  by  the  end  of  the 
mercury  column.  The  division  reached 
gives  the  temperature  directly. 

The  centigrade  scale  just  described  is 
the  one  commonly  used  by  scientists  the 
world  over,  but  engineers  in  this  country 
more  often  use  what    is    known  as   the 
FIG  ^-Comparison  Fahrenheit  scale.     This  is  so  chosen  that 
of  Centigrade  and    ,,  -         ,,. 

Fahrenheit  Scales.   the  temperature  of  melting  ice  is  called 
32°  F.  and  the  temperature  of  water  boil- 
ing under  atmospheric  pressure  is  called  212°  F.     There  are 


c.' 


17.8 


12 


STEAM  POWER 


thus  180°  on  this  scale  for  the  same  temperature  difference 
as  is  represented  by  100°  on  the  centigrade  scale.  The 
relation  between  the  two  scales  is  shown  diagrammatically 
in  Fig.  6.  It  is  apparent  that  the  temperature  of  a  body 
at  0°  C.  will  be  32°  F.  and  that  of  a  body  at  0°  F.  will  be 
-17.8CC. 

Since  100  centigrade  degrees  are  equal  to  180  Fahren- 
heit degrees,  it  follows  that 


i  o  (^       AUV^       ^  o  T7< 

1  c    Too    5    F' 


and  that 


1  o  -p     100     5  o  n 
1    F  =  l80  =  9     C' 


(1) 


(2) 


Therefore,  if  tF  and  tc  represent  temperatures  on  the  Fahren- 
heit and  centigrade  scales  respectively, 


and 


(3) 
(4) 


Cent  . 


Fahr. 


100 


-273 


459.4 


Ma- 


is  still  another  temperature  scale  of  great  impor- 
tance. It  is  known  as  the  absolute 
scale  and  temperatures  measured 
on  it  are  spoken  of  as  absolute  tem- 
peratures. The  zero  on  this  scale 
is  located  at  -273°  C.  or  273  centi- 
grade degrees  below  centigrade 
zero,  or,  what  is  the  same  thing, 
at  -459.4°  F.,  or  459.4  Fahrenheit 
degrees  below  Fahrenheit  zero.  The 
degrees  used  are  either  centigrade 

FIG.  7. — Comparison  of  Ab-  or  Fahrenheit,  as  convenient,  so 
solute  and  Ordinary  that  there  are  absolute  tempera- 
tures expressed  in  centigrade  de- 

grees  above  absolute  zero  and  there  are  absolute  tempera- 


459.4° 


PHYSICAL  CONCEPTIONS  AND  UNITS  13 

tures  expressed  in  Fahrenheit  degrees  above  absolute  zero. 
The  relations  between  the  various  scales  are  shown  dia- 
grammatically  in  Fig.  7. 

It  is  apparent  from  this  diagram  that, 

TV  =  ^+460  (approximately)     .       ...'..  (5) 
and  that 

f     ......  ".     J     (6) 


if  TF  and  Tc  represent  absolute  temperatures  and  if  the 
number  459.4  is  rounded  out  to  460,  as  is  commonly 
done. 

9.  The  Unit  of  Heat  Energy.  The  unit  used  in  the 
measurement  of  heat  energy  in  the  United  States  is  the 
British  Thermal  Unit  (abbreviated  B.t.u).  It  is  defined  as 
the  quantity  of  heat  required  to  raise  the  temperature  of  one 
pound  of  pure  water  one  degree  Fahrenheit.  In  order  to 
make  the  definition  very  exact  it  is  necessary  to  state  the 
temperature  of  the  water  before  the  temperature  rise  occurs, 
because  it  requires  different  amounts  of  heat  to  raise  the 
temperature  of  a  pound  of  water  one  degree  from  differ- 
ent initial  temperatures.  For  ordinary  engineering  pur- 
poses, however,  such  refinements  generally  may  be  omitted. 

Many  experimenters  have  shown  that  heat  energy  and 
mechanical  energy  are  mutually  convertible,  that  is,  the  one 
can  be  changed  into  the  other.  When  such  a  change  occurs 
no  energy  can  be  lost  since  energy  is  indestructible,  and  it 
follows  that,  if  one  form  is  changed  into  the  other,  there 
must  be  just  as  much  energy  present  after  the  change  as 
there  was  before. 

As  the  units  used  in  measuring  the  two  forms  of  energy 
are  very  different  and  as  it  is  often  necessary  to  express 
quantities  of  energy  taking  part  in  such  conversions,  it  is 
desirable  to  determine  the  relations  between  these  units. 
This  was  first  accurately  done  by  Joule,  who  showed  that  one 
British  thermal  unit  of  heat  energy  resulted  from  the  con- 


14  STEAM  POWER 

version  of  772  ft.-lbs.  of  mechanical  energy.  Later  experi- 
menters have  shown  that  the  number  778  more  nearly 
expresses  the  truth  than  does  the  number  772  and  the  larger 
value  is  now  known  as  Joule's  Equivalent. 

Expressed  mathematically,  the  relation  between  the  units 
is 

lB.t.u.  =  778  ft.-lbs  ......     (7) 


(8) 


10.  Specific  Heat.  The  specific  heat  of  a  substance  is 
defined  as  that  quantity  of  heat  which  is  used  up  or  recovered 
when  the  temperature  of  one  pound  of  the  material  in  question 
is  raised  or  lowered  one  degree.  Its  numerical  value  depends 
upon  the  specific  heat  of  water  since  the  quantity  of  heat  is 
measured  in  units  dependent  upon  the  amount  required  to 
raise  the  temperature  of  water.  The  specific  heat  of  water 
is,  however,  very  variable,  as  shown  by  the  values  given  in 
Table  I.,  and  it  is  therefore  evident  that  exact  numerical 
values  of  specific  heats  can  only  be  given  when  the  definition 
of  the  B.t.u.  is  exactly  expressed. 

The  specific  heats  of  all  real  substances  vary  with  tem- 
perature and  the  values  commonly  used  are  either  rough 
averages  or  are  those  determined  by  experiments  at  one 
temperature.  For  most  engineering  purposes  errors  arising 
from  this  source  may,  however,  be  neglected. 

From  the  definition  of  specific  heat  it  follows  that  : 


in  which 


C  =  a  mean  or  average  specific  heat  over  a  range  of  tem- 
perature from  ti  to  <2,  and 

Q  =  the  heat  supplied  to  raise  the  temperature  of  W 
pounds  of  material  from  t\  to  £2. 


PHYSICAL  CONCEPTIONS  AND  UNITS 


15 


TABLE  I 
SPECIFIC  HEATS  OF  WATER.* 

(Value  at  55°  F.  taken  as  unity) 


Temp.  F°. 

Spec.  Ht. 

Temp.  F°. 

Spec.  Ht. 

20 

1.0168 

350 

1.045 

30 

1.0098 

400 

.064 

40 

1.0045 

450 

.086 

50 

1.0012 

500 

.112 

60 

0.9990 

510 

.117 

•   70 

0.9977 

520 

.123 

80 

0.9970 

530 

.128 

90 

0.9967 

540 

.134 

100 

0.9967 

550 

.140 

120 

0.9974 

560 

.146 

140 

0.9986 

570 

.152 

160 

1.0002 

580 

.158 

180 

1.0019 

590 

.165 

200 

1.0039 

600 

.172 

220 

1.007 

240 

1.012 

260 

1.018 

280 

1.023 

300 

1.029 

*  Values  taken  from  Marks  and  Davis,  "  Steam  Tables  and  Diagrams,"  p.  68. 

ILLUSTRATIVE   PROBLEMS 

1.  Given:  Sp.  ht.  of  iron=0.113,  of  aluminum  =0.211;  Initial 
temp.  =  150°  F.  Temp,  range  (fe  -ti)  =  100°  F. 

If  1  Ib.  of  iron  and  1  Ib.  of  aluminum  are  cooled  through  this 
temperature  range,  how  much  more  heat  is  lost  in  one  case  than 
in  the  other? 


Qlr 


21.1  B.t.u. 
1  X  .113X  100  =  11.3  B.t.u. 


Difference  9.8  B.t.u. 


2.  If  the  difference  obtained  in  Prob.  1  were  used  to  heat  up 
5  Ibs.  of  silver,  with  a  specific  heat  equal  to  0.057,  what  would  be 
the  temperature  range  through  which  it  would  be  raised? 

Q  =9.8  =5X0.057(«2  -fc)  =0.285(fe-«i) 


16  STEAM  POWER 

3.  If  the  initial  temperature  of  the  silver  in  Prob.  2  were  150°  Ft 
what  would  be  the  final  absolute  temperature  Fahr.? 

t2  =  ^+34.4°  =  150+34.4  =  184°  (approximately) . 
r,  =460+184  =644°  F.  Abs. 

4.  100  Ibs,  of  water  in  a  20-lb.  tank  of  iron,  both  at  60°  F., 
are  placed  in  salt  brine  at  0°  F.    The  water  becomes  ice  at  32°  F. 
and  the  temperature'of  the  ice  is  lowered  to  26°  F.,  the  brine  being 
raised  to  26°  F.     Sp.  ht.  water  =  1.0;    Sp.  ht.    ice  =0.5;    Sp.  ht. 
iron  =0.113;  Sp.  ht.  brine  =0.8;  and  143  B.t.u.  per  pound  of  water 
must  be  removed  to  convert  liquid  water  at  32°  F.  to  ice  at  the 
same  temperature.     What  weight  of  brine  is  required? 

100[l(60-32)  +  143+.5(32-26)]+20X0.113(60-26) 

=  TFX0.8(26-0) 
TF=840  Ibs.  of  brine. 

11.  Quantity  of  Heat.  It  is  impossible  to  determine  the 
total  quantity  of  heat  in  or  "  associated  with  "  a  substance, 
because  no  means  of  removing  and  measuring  all  the  heat 
contained  in  any  real  material  have  ever  been  devised. 
Since,  however,  the  engineer  is  concerned  with  changes  of 
heat  content  rather  than  with  the  total  amount  of  heat 
contained,  this  fact  causes  him  no  difficulty. 

For  convenience  in  figuring  changes  of  heat  content, 
it  is  customary  to  assume  some  arbitrary  starting  point  or 
datum  and  to  call  the  heat  in  the  material  in  question  zero 
at  that  point. 

Thus,  for  example,  if  it  were  necessary  to  figure  heat 
changes  experienced  by  a  piece  of  iron  weighing  5  Ibs.  and 
having  a  specific  heat  of  0.1138,  and  the  temperature  of  this 
iron  never  dropped  below  40°  F.  under  the  conditions  exist- 
ing, this  temperature  might  be  taken  as  an  arbitrary  starting 
point  above  which  to  figure  heat  contents.  If  the  iron  were 
later  found  at  a  temperature  of  75°  F.,  •"  the  heat  content 
above  40°  F."  would  be  said  to  be 

Q  =  CW  (t2-ti)  =0.1138X5(75-40)  =2.27  B.t.u. 


PHYSICAL  CONCEPTIONS  AND  UNITS  17 

This  type  of  formula  can  only  be  used  when  the  sub- 
stance does  not  change  its  state  between  the  limits  of  tem- 
perature concerned.  In  the  case  of  water  which  might 
change  to  steam  during  such  a  rise  of  temperature,  it  might 
be  necessary  to  include  other  heat  quantities  in  the  cal- 
culations, as  shown  in  a  later  chapter. 

12.  Work  and  Power.  Since  steam  engines  are  designed 
for  the  purpose  of  converting  the  heat  energy  contained  in 
fuel  into  mechanical  energy  which  may  be  used  to  perform 
work,  it  will  be  necessary  to  consider  the  units  used  in 
measuring  work  and  power. 

Work  was  defined  in  a  previous  paragraph  as  the  over- 
coming of  a  resistance  through  a  distance,  by  the  application 
of  a  force;  that  is,  a  force  expressed  in  pounds,  multiplied  by 
the  distance  in  feet  through  which  the  force  acts,  gives  a  product 
expressed  in  foot-pounds. 

The  amount  of  work  performed  in  a  unit  of  time  is  termed 
power,  which  may  be  denned  as  the  rate  of  doing  work. 
Therefore, 

Force  X  Distance 

Power  =  =7-      —^-         -^.      .     .     .     (10) 
Time  (mm.  or  sec.) 

The  unit  of  power  used  by  steam  engineers  is  the  horse- 
power, which  is  equivalent  to  the  performance  of  33,000 
ft.-lbs.  of  work  per  minute,  or  550  ft.-lbs.  of  work  per  second, 
or  1,980,000  ft.-lbs.  per  hour.  Therefore,  the  horse-power 
developed  by  any  mechanism  is 

ft.-lbs.  of  work  per  min. 

h'p-= 


Since  33,000  ft.-lbs.  of  work  can  be  accomplished  only  by 
the  expenditure  of  33,000  ft.-lbs.  of  energy  and  since  one 
B.t.u.  of  energy  is  equal  to  778  ft.-lbs.,  it  follows  that  33,000 


oo 
ft.-lbs.  of  work  must  be  the  equivalent  of      *Q    =42.41  B.t.u. 

4   4  O 

It  is  customary  to  speak  of  power  in  terms  of  horse- 


18  STEAM  POWER 

power-hours.  One  horse-power-hour  means  the  doing  of 
work  equivalent  to  one  horse-power  for  the  period  of  one 
hour,  or  the  doing  of  work  at  the  rate  of  33,000  ft.-lbs.  per 
minute  for  an  hour.  A  horse-power-hour  is  therefore  equiva- 
lent to  33,000X60=1,980,000  ft.-lbs.  As  33,000  ft.-lbs. 
are  equivalent  to  42.41  B.t.u.,  it  follows  that  42.41X60  = 
2544.6  or  about  2545  B.t.u.  are  the  equivalent  of  one  horse- 
power-hour. 

The  number  2545  should  be  memorized  as  it  is  very  often 
used  in  steam-power  calculations.  If  an  engine  could  deliver 
one  horse-power-hour  for  every  2545  B.t.u.  it  received,  it 
would  be  working  without  losses  of  any  kind ;  that  is,  all 
the  heat  energy  entering  it  would  leave  it  in  the  form  of 
useful  mechanical  energy.  It  will  be  shown  later  that  this 
is  impossible  even  in  the  most  perfect  or  ideal  engine. 

REVIEW    PROBLEMS 

1.  Express  32°  F.  in  degrees  centigrade. 

2.  Express  150°  F.  in  degrees  centigrade. 

3.  Express  250°  C.  in  degrees  Fahrenheit. 

4.  Express  the  results  of  problems  1,  2  and  3  in  absolute  values. 

5.  What  is  the  heat  equivalent  of  233,400  ft.-lbs.  of  work? 

6.  Find  the  heat  supplied  10  Ibs.  of  water  when  its  temperature 
is  raised  from  20°  F.  to  160°  F.,  assuming  the  mean  specific  heat 
over  this  range  to  be  0.997. 

7.  Find  the  temperature  change  of  2  Ibs.  of  lead  (sp.  ht.  0.0314) 
when  20  B.t.u.  are  added. 

8.  How  many  B.t.u.  must  be  abstracted  to  lower  the  tem- 
perature of  15  Ibs.  of  water  from  212°  F.  to32°F.,  assuming  the 
specific  heat  of  water  to  be  unity? 

9.  Find  the  weight  of  water  which  will  have  its  temperature 
tripled  in  value  by  the  addition  of  250  B.t.u.,  the  final  temperature 
being  150°  F.     Assume  specific  heat  unity. 

10.  The  specific  heat  of  a  piece  of  wrought  iron  is  0.113  and  of 
a  given  weight  of  water  is  1.015.     1  cu.  ft.  of  water  weighs  approxi- 
mately 62.5  Ibs.     Find  the  increase  in  temperature  of  4  cu.  ft.  of 
water  when  a  common  temperature  of  65°  F.  results  from  placing 
in  the  water  a  piece  of  iron  weighing  15  Ibs.  at  a  temperature  of 
900°  F. 


PHYSICAL  CONCEPTIONS  AND  UNITS  19 

11.  Find  the  final  temperature  of  the  mixture,  when  100  Ibs. 
of  iron  (sp.  ht.  =0.113),  at  a  temperature  of  1200°  F.  are  immersed 
in  300  Ibs.  of  water  (sp.  ht.  1.001)  at  a  temperature  of  50°  F. 

12.  Five  pounds  of  silver  (sp.  ht.  =  0.057)  at  800°  F.  are  im- 
mersed in  water  at  60°  F.,  resulting  in  a  final  temperature  of  85°  F. 
Assume  Sp.  ht.  water  =  1.    What  weight  of  water  is  necessary? 

13.  An  engine  is  developing  10  horse-power.     Express  this  in 
ft  .-Ibs.  of  work   done   per  minute  and  find  the  amount  of  heat 
energy  equivalent  to  this  quantity  of  mechanical  energy. 

14.  A  pump  raises   1000  Ibs.  of  water  50  ft.  every  minute. 
How  much  work  is  done?     Find  the  equivalent  horse-power. 

15.  An  engine  develops  1,980,000  ft.-lbs.  of  work  at  the  fly- 
wheel per  minute. 

(a)  Find  the  horse-power  developed. 

(6)  If  this  engine  operated  in  this  way  for  an  hour,  how  many 
horse-power  hours  would  it  make  available? 

(c)  What  would  be  the  equivalent  of  this  number  of  horse- 
power hours  in  British  thermal  units? 


CHAPTER   II 
THE  HEAT-POWER  PLANT 

13.  The  Simple  Steam-Power  Plant.  The  various  pieces 
of  apparatus  necessary  for  the  proper  conversion  of  heat 
energy  into  mechanical  power  constitute  what  may  be 
called  a  "  Heat-Power  Plant,"  just  as  the  apparatus  used 
in  obtaining  mechanical  energy  from  moving  water  is  called 
an  hydraulic  or  water-power  plant.  Heat-power  plants  are 
distinguished  as  "Steam-Power  Plants";  "Gas-Power 
Plants  ";  etc.,  according  to  the  way  in  which  the  heat  of 
the  fuel  happens  to  be  utilized. 

The  apparatus  around  which  the  plant  as  a  whole  centers, 
that  is,  the  apparatus  in  which  heat  energy  is  received  and 
from  which  mechanical  energy  is  delivered,  is  termed  the 
engine  or  prime-mover.  This  heat  engine  may  use  steam 
generated  in  boilers  and  may  require  certain  apparatus,  such 
as  condensers,  pumps,  etc.,  for  proper  operation;  or  it  may 
use  gas,  generated  in  gas-producers  requiring  coolers, 
scrubbers,  tar  extractors  and  holders,  depending  upon  the 
class  of  fuel  used  and  upon  certain  commercial  considera- 
tions. Again,  the  power-plant  may  simply  contain  an  in- 
ternal-combustion engine  using  natural  gas,  gasoline  or  oil, 
a  type  of  plant  which  is  now  very  common. 

But  whatever  type  of  plant  is  used,  a  general  method  of 
operation  is  common  to  all.  Heat  energy  in  fuel  is  constantly 
fed  in  at  one  end  of  the  system  and  mechanical  energy  is 
delivered  at  the  other  end.  The  steam-power  plant  will  be 
briefly  described  in  the  following  paragraphs,  showing  the 
cycle  of  events  with  the  attendant  losses  through  the 
system. 

20 


THE  HEAT-POWER  PLANT 


21 


22  STEAM  POWER 

In  Fig.  8  is  shown  a  simple  steam-power  plant  which  con- 
verts into  mechanical  energy  part  of  the  heat  energy,  origi- 
nally stored  in  coal,  by  means  of  a  prime-mover  called  a 
steam-engine.  The  main  pieces  of  apparatus  used  in  this 
type  of  plant  are  the  steam-boiler;  the  steam-engine;  the 
condenser;  the  vacuum  pump;  and  the  feed-pump.  The 
energy  stream  shows  the  various  losses  occurring  through- 
out the  plant.  These  losses  cause  the  "  delivered  energy  " 
stream  to  be  only  a  small  fraction  of  the  total  heat  sent  into 
the  system. 

14.  Cycle  of  Events.  1.  Fuel  is  charged  on  the  grate 
under  the  boiler,  where  it  is  burned  with  the  liberation  of  a 
large  amount  of  energy.  Air  is  drawn  or  forced  through 
the  grates  in  proper  proportions  to  support  this  combustion. 
The  hot  gases  resulting  pass  over  the  tubes,  in  a  definite 
path  set  by  the  baffle  plates,  so  that  the  largest  possible 
amount  of  heating  surface  may  be  presented  to  the  products 
of  combustion. 

There  are  certain  losses  accompanying  this  operation, 
such  as  radiation,  loss  of  volatile  fuel  passing  off  unburned, 
loss  of  fuel  through  the  grate,  and  loss  of  heat  through  the 
excess  air  which  must  always  be  supplied  to  insure  com- 
bustion. 

2.  That  part  of  the  heat  in  the  gases  which  is  not  lost 
by  radiation  from  the  boiler  and  in  the  hot  gases  flowing  up 
the  stack  passes  through  the  heating  surfaces  of  the  boiler 
to  the   water    within.     From  50   to    80    per   cent   of   the 
total  heat   energy  in  the  fuel  passes  through  the  heating 
surfaces  and  serves  to  raise  the  temperature  of  the  water 
to  the  boiling  point  at  the  pressure  maintained,  and  to  con- 
vert this  water  into  steam  according  to  the  requirements. 

3.  Having  obtained  steam  within  the  boiler,  it  is  led 
through  a  system  of  pipes  to  a  steam  engine,  where  some  of 
the  heat  stored  in  the  steam  is  converted  into  mechanical 
energy  by  the  action  of  that  steam  against  a  piston.     The 
steam  is  then  discharged,  or  exhausted,  from  the  engine 


THE  HEAT-POWER  PLANT  23 

at  a  much  lower  temperature  and  pressure  than  when  it 
entered. 

From  5  to  22  per  cent  of  the  available  heat  in  the 
steam  is  converted  into  mechanical  energy  in  the  engine 
cylinder,  and  because  of  frictional  and  other  losses  occurring 
in  the  mechanism,  only  from  85  to  95  per  cent  of  this 
energy  is  turned  into  useful  work  at  the  fly-wheel. 

4.  In  some  plants,  known  as  non-condensing  plants, 
the  exhaust  steam,  which  still  contains  the  greater  part  of 
all  the  heat  received  in  the  boiler,  is  discharged  to  the  atmos- 
phere and  represents  a  complete  loss.  In  others,  known  as 
condensing  plants,  the  exhaust  steam  is  led  to  a  condenser, 
where  it  is  condensed  by  cold  water,  which  absorbs  and 
carries  away  the  greater  quantity  of  the  heat  not  utilized 
in  the  engine.  The  condensed  steam  or  "  condensate  "  is 
then  either  discharged  to  the  sewer  or  transferred  by  means 
of  a  vacuum-pump  to  the  hot-well,  from  which  it  is  drawn 
by  means  of  the  feed-water-pump,  raised  to  the  original 
pressure  of  the  steam,  and  returned  to  the  boiler.  Here 
it  is  again  turned  into  steam  and  the  cycle  of  operations 
outlined  above  is  repeated.  Naturally  there  is  some  loss 
due  to  evaporation  and  leaks  throughout  the  system,  so 
that  "  make-up  "  water  must  constantly  be  supplied. 

The  series  of  events  just  described  constitutes  a  complete, 
closed  cycle  of  operations,  wherein  the  water  is  heated, 
vaporized,  condensed  and  returned  to  the  boiler,  having 
served  only  as  a  medium  for  the  transfer  of  heat  energy 
from  fuel  to  engine  and  the  conversion  of  part  of  that 
energy  within  the  cylinder.  The  water  in  such  a  case  is 
known  as  the  working  substance. 

It  is  often  more  convenient  to  discard  the  working  sub- 
stance after  it  leaves  the  cylinder,  as  suggested  above  in 
the  case  of  a  non-condensing  plant;  or,  as  in  the  case  of 
a  gas  engine,  where  a  new  supply  of  working  substance 
must  be  supplied  for  each  cycle,  because  the  burned  gases 
of  the  previous  cycle  cannot  be  used  again. 


24  STEAM  POWER 

15.  Action  of  Steam  in  the  Cylinder.  In  order  to  pre- 
pare for  the  more  detailed  discussion  of  the  action  of  the 
steam  in  the  engine  cylinder,  to  be  taken  up  in  a  later 
chapter,  a  brief  outline  of  the  events  occurring  within  the 
prime-mover  will  be  considered  at  this  point. 

Steam  enters  the  cylinder  through  some  kind  of  an 
admission  valve,  and  acts  upon  the  piston,  just  as  the  latter 
has  approximately  reached  one  end  of  its  stroke  and  is 
ready  to  return.  The  heat-energy  stored  up  in  the  steam 
causes  it  to  expand  behind  the  piston,  thereby  driving  the 
latter  out  and  performing  work  at  the  fly-wheel.  At  about 
90  or  95  per  cent  of  the  stroke,  the  exhaust  valve  opens, 
and  the  steam  begins  to  exhaust,  the  pressure  within  the 
cylinder  dropping  almost  to  atmospheric  or  to  that  main- 
tained in  the  condenser  by  the  time  the  piston  has 
reached  the  end  of  its  stroke.  On  the  next  or  return  stroke 
the  remaining  steam  is  forced  out  through  the  exhaust 
port,  until,  at  some  point  before  the  end  of  the  piston 
travel,  the  exhaust  valve  closes,  and  the  low-pressure  steam 
trapped  in  the  cylinder  is  compressed  into  the  clearance 
space  so  that  its  pressure  rises.  Admission  then  occurs,  and 
the  cycle  is  repeated. 

The  diagram  given  in  Fig.  9  illustrates  the  operation  of 

steam  within  the  cylinder. 
This  diagram  is  plotted 

Cut-off  (Closing  of  Admission  Valve) 

between  pressures  01  steam 
within  the  cylinder  as 

Release  (Opening  of  -• .  ••  j 

Exhaust  vaive)       ordmates  and  correspond- 
f\  ing    piston    positions    as 

abscissas, 
piston  positions  The  method  of  obtain- 

Closing  of  Exhaust  Valve 

^  c,        ,-,     .  ing  such  a  diagram,  known 

FIG.  9. — Steam  Engine  Indicator  ' 

Diagram,  as  an   indicator-diagram, 

will  be  fully  described  in  a 

later  chapter.     Since  vertical  ordinates  represent  pressure 
in  pounds  per  square  inch,  and  horizontal  abscissas  renre- 


THE  HEAT-POWER  PLANT 


25 


sent  feet  moved  through  by  the  piston,  the  product  of 
these  two  must  be  work.  But  the  product  of  vertical  by 
horizontal  distances  must  also  give  area.  Therefore,  by 


Source  of  Water 
at  High  Head 


Energy  Supplied 


iver  of  Discharged 
Water  at  Low  Head 


Head  ft  2 
above;  datum 


(3) 


Source  of  Heat 
at  High  Temp. 


High  Temp, 
tj  above  datum 


Energy  Supplied 


LowJTemp. 
tg above  datura 


Useful  Energy- 
Made  Available 


Energy  Discharged 

eiver  of  Discharged 
Heat  at  Low 
Temperature 


FIG.  10.—  Hydraulic  Analogy. 

finding  the  area  enclosed  within  the  bounding  lines  of  the 
cycle  and  multiplying  this  by  a  proper  factor,  the  foot- 
pounds of  work  developed  within  the  cylinder  can  be 
determined. 

16.  Hydraulic  Analogy.     The  operation  of  heat-engines 
is  analogous  to  that  of  water-wheels.     A  water-wheel  de- 


26  STEAM  POWER 

velops  mechanical  energy  by  receiving  water  under  a  high 
head,  absorbing  some  of  its  energy,  and  then  rejecting  the 
fluid  under  a  low  head.  Similarly,  the  heat-engine  receives 
heat  energy  at  a  high  temperature  (head),  absorbs  some  of 
it  by  conversion  into  mechanical  energy,  and  then  rejects 
the  rest  at  a  low  temperature  (head). 

The  analogy  can  be  carried  still  further.  The  water- 
wheel  cannot  remove  all  the  energy  from  the  water,  nor 
can  the  heat-engine  remove  all  the  heat-energy  from  the 
working  substance.  There  is  a  certain  loss  in  the  material 
discharged  in  both  cases  and  this  cannot  be  avoided. 

This  analogy  is  illustrated  diagrammatically  in  Fig.  10 
(a)  and  (6)  in  which  the  widths  of  the  streams  represent 
quantity  of  energy. 


CHAPTER   III 
STEAM 

17.  Vapors  and  Gases.     When  a  solid  is  heated,  under 
the  proper  pressure  conditions,  it  ultimately  melts  or  fuses 
and   becomes   a   liquid.     The   temperature   at   which   this 
occurs  depends  upon  the  particular  material  in  question 
and  upon  the  pressure  under  which  it  exists.     Ice,  which 
is  merely  solid  water,  melts  at  32°  F.  under  atmospheric 
pressure,  while  iron  melts  at  about  2000°  F.  under  atmos- 
pheric pressure. 

When  a  liquid  is  heated,  it  ultimately  becomes  a  gas, 
similar  to  the  air  and  other  familiar  gases.  If  this  gas  is 
heated  to  a  very  high  temperature  and  if  the  pressure  under 
which  it  is  held  is  not  too  great,  it  very  nearly  obeys  certain 
laws  which  are  simple  and  which  are  called  the  laws  of  ideal 
gases. 

When  the  material  is  in  a  state  between  that  of  a  liquid 
and  that  in  which  it  very  nearly  obeys  the  laws  of  ideal 
gases,  it  is  generally  spoken  of  as  a  vapor.  This  term  is 
used  in  several  different  ways  and  with  several  different 
modifying  adjectives  which  will  be  explained  in  greater 
detail  in  later  sections. 

18.  Properties  of  Steam.     Of  the  many  vapors  used  by 
the  engineer,  steam  or  water  vapor  is  probably  the  most 
important,  because  of  the  ease  with  which  it  can  be  formed 
and  also  because  of  the  tremendous  field  in  which  it  can 
be  used.     It  is  generated  in  a  vessel  known  as  a  steam  boiler, 
which  is  constructed  of  metal  in  such  a  way  that  it  can 
contain  water,  and  that  heat  energy,  liberated  from  burning 
fuel,  can  be  passed  into  the  water,  converting  part  or  all  of 
it  into  water  vapor,  that  is,  into  steam. 


28 


STEAM  POWEE 


The  properties  of  water  vapor  must  be  thoroughly  under- 
stood before  the  steam  engine  and  steam  boiler  can  be 
studied  profitably.  Probably  the  easiest  way  of  becoming 
familiar  with  these  properties  is  to  study  the  use  made  of 
heat  in  the  generation  of  steam  from  cold  water. 

19.  Generation  of  Steam  or  Water  Vapor.  For  the  pur- 
poses of  development,  assume  a  vessel  of  cylindrical  form, 
fitted  with  a  frictionless  piston  of  known  weight,  as  shown 
in  Fig.  11,  (a)  and  (6),  the  whole  apparatus  being  placed 
under  a  bell-jar  in  which  a  perfect  vacuum  is  maintained. 


/ 

\ 

Illl 

nil!,! 

M 

F     "| 

%& 

t 

(b) 

__:_/ 

iZE 

L 

c—  c: 

J 

////// 

//////, 

///// 

To  Vacuum 
Pump 

FIG.  11. — Formation  of  Steam  at  Constant  Pressure. 


Assume  one  pound  of  water  in  the  cylinder,  with  the  piston 
resting  on  the  surface  of  the  liquid.  There  will  be  some 
definite  pressure  exerted  by  the  piston  upon  the  surface  of 
the  liquid,  and  its  value  will  be  determined  entirely  by  the 
weight  of  the  piston. 

It  is  convenient  in  engineering  practice  to  refer  all 
vaporization  phenomena  to  some  datum  temperature,  and 
since  the  melting  point  of  ice,  32°  F.,  is  a  convenient  refer- 
ence point,  it  is  used  as  a  standard  datum  temperature,  in 
practically  all  steam-engineering  work.  Therefore,  assuming 
the  water  in  the  jar  to  be  at  32°  F.,  if  heat  is  applied  the 
temperature  of  the  liquid  will  rise  approximately  1°  F. 


STEAM 


29 


for  every  B.t.u.  of  heat  added,  since  the  specific  heat  of 
water  is  approximately  unity. 

Experiment  shows  that  for  each  pressure  under  which  the 
water  may  exist  some  definite  temperature  will  be  attained  at 
which  further  rise  of  temperature  will  cease  and  the  liquid  will 


480 
440 

•g  400 
a 


360 


£240 

i  m 

£  1GO 

120 

80 

40 


0        40        80       120      ItiO      200     240      280      320      3UO     400      440      480      520 
Temperature  -F.° 

FIG.  12.— Pressure-Temperature  Relations,  Saturated  Water  Vapor. 

begin  to  change  to  a  vapor,  that  is,  to  vaporize.  The  tem- 
peratures at  which  vaporization  occurs  at  different  pressures 
are  called  the  temperatures  of  vaporization  at  those  pressures. 
The  temperatures  of  vaporization  of  water  are  plotted 
against  pressure  in  Fig.  12.  It  should  be  noted  that  the 
values  of  vaporization  temperature  increase  very  rapidly 
for  small  pressure  changes  in  the  case  of  low  pressures,  but 


30  STEAM  POWER 

that,  for  the  higher  pressures,  the  variation  of  temperature 
is  very  small  for  enormous  variations  of  pressure.  This 
fact  is  of  great  importance  in  steam  engineering. 

The  temperatures  of  vaporization  are  tabulated  with 
other  properties  of  water  vapor  in  so-called  steam  tables  and 
are  constantly  referred  to  by  engineers.  An  example  of 
such  a  table  is  given  on  pp.  392  to  399. 

Returning  now  to  the  apparatus  under  discussion,  as 
heat  is  supplied,  the  temperature  of  the  water  will  rise  from 
32°  F.  until  it  reaches  the  temperature  of  vaporization  cor- 
responding to  the  pressure  exerted  upon  the  water  by  the 
piston.  When  this  temperature  is  reached  vaporization  will 
begin,  and  if  sufficient  heat  is  supplied,  will  continue  without 
change  of  temperature  until  the  water  is  entirely  converted 
into  vapor. 

Up  to  the  time  at  which  vaporization  starts  the  volume 
of  the  water  will  change  very  little,  so  that  the  piston  will 
be  raised  only  a  negligibly  small  amount  and  practically  no 
work  will  be  done  upon  it  by  the  water.  On  the  other  hand, 
when  vaporization  occurs  the  volume  of  the  material  will 
change  by  a  very  large  amount  and  the  piston  will  be 
driven  out  (raised)  against  the  action  of  gravity.  That  is, 
work  will  be  done  by  the  steam  in  driving  the  piston  out 
during  the  increase  in  volume  which  accompanies  vaporiza- 
tion. 

It  is  found  that  a  very  great  quantity  of  heat  is  used  up 
during  the  process  of  vaporization  despite  the  fact  that  no 
temperature  change  occurs.  This  is  described  by  saying  that 
the  heat  which  is  supplied  during  this  period  becomes  latent 
in  the  steam  formed,  and  the  quantity  of  heat  is  therefore 
spoken  of  as  the  latent  heat  of  vaporization.  It  is  assumed 
to  consist  of  two  parts,  that  used  for  separating  the  liquid 
molecules  against  their  attractive  forces  and  that  used  for 
doing  the  work  which  is  done  upon  the  piston  as  it  is 
moved  upward.  The  former  is  called  the  internal  latent 
heat  because  it  is  used  for  doing  internal  or  intermolecular 


STEAM  31 

work;  the  latter  is  called  external  latent  heat  because  it  is 
used  for  the  doing  of  external  work. 

It  is  to  be  noted  that  the  internal  latent  heat  may  be 
assumed  to  be  tied  up  in  some  way  within  the  molecular 
structure  of  the  material  and  hence  to  be  in  the  steam. 
The  external  latent  heat,  on  the  other  hand,  is  used  up  as 
fast  as  supplied  for  the  purpose  of  driving  the  piston  out 
against  the  action  of  gravity.  When  the  piston  has  been 
raised  to  any  point,  the  energy  used  in  raising  it  is  not  in 
the  steam,  but  is  stored  as  potential  energy  in  the  piston. 
To  get  it  back  the  piston  must  be  allowed  to  drop.  The 
term  "  external "  is  therefore  well  chosen;  the  external 
latent  heat  is  in  no  sense  in  the  steam;  it  is  stored  in 
external  bodies  or  mechanism. 

After  the  constant  temperature  vaporization  is  complete, 
the  further  addition  of  heat  will  again  cause  a  rise  of  tem- 
perature and  a  gradual  increase  of  volume.  Such  raising 
of  the  temperature  of  steam  already  formed  is  called  super- 
heating and  results  in  carrying  the  vapor  nearer  and  nearer 
to  the  condition  in  which  it  very  nearly  obeys  the  laws  of 
ideal  gases.  Since  an  increase  of  volume  accompanies  super- 
heating, the  molecules  of  the  vapor  must  move  farther  and 
farther  apart  as  superheating  progresses. 

Vapor  in  the  condition  in  which  it  is  formed  from  the 
liquid  and  which  has  the  same  temperature  as  the  liquid 
from  which  it  was  formed  is  called  saturated  vapor.  This 
term  can  be  pictured  as  meaning  that  the  maximum  number 
of  molecules  of  vapor  are  packed  into  a  given  space;  the 
addition  of  heat  to  saturated  vapor  would  cause  superheating 
and  the  separation  of  the  molecules  so  that  fewer  could  be 
contained  in  a  given  space. 

20.  Heat  of  Liquid,  q  or  h.  Returning  once  more  to 
the  start-  of  the  process  described  in  the  preceding  section, 
heat  was  added  to  water  initially  at  32°  F.  until  the  tem- 
perature of  vaporization  corresponding  to  the  existing  pres- 
sure was  attained.  The  heat  added  during  this  period  is 


32  STEAMj  POWER 

called  the  heat  of  the  liquid,  and  is  usually  designated  by  the 
letters  q  or  h.  If  the  mean  specific  heat  of  water  at  con- 
stant pressure  (Cp)  for  the  temperature  range  under  con- 
sideration were  constant,  and  equal  to  1,  then,  since 


in  which  tv  is  the  temperature  of  vaporization,  it  would 
follow  that,  for  this  pressure 

q  =  tv-32  ........     (12) 


Therefore,  if  water  boils  under  a  pressure  of  50  Ibs.  at 
a  temperature,  read  from  the  steam  tables,  of  281°  F.;  it 
would  follow  that 


But  the  steam  tables  (see  p.  394)  for  this  pressure  (50 
Ibs.)  give  5  =  250.1  B.t.u.,  indicating,  as  was  shown  in 
Chap.  I.,  that  the  specific  heat  of  water  does  not  remain 
constant,  and  for  this  case  the  mean  value  must  have  been 
approximately  1.004  as  indicated  by  the  following  calcula- 
tion. 

q  =  Cp  (k-32)  or 
so  that 

^250.1 
p      249 

Hence  it  is  always  advisable  to  use  the  steam  table 
values  of  q,  except  for  very  approximate  calculations. 

21.  Latent  Heat  of  Vaporization,  r  or  L.  The  heat 
supplied  during  the  period  of  vaporization  has  already  been 
referred  to  as  the  latent  heat  of  vaporization  and  has  been 
divided  into  internal  and  external  latent  heats. 

The  internal  latent  heat  is  generally  designated  by  p  or 
by  7  and  the  external  latent  heat  by  the  group  of  letters 
APu  or  by  E.  The  group  APu  merely  represents  the  prod- 


STEAM  33 

uct  of  pressure,  P,  by  volume  change  during  vaporization, 
u,  and  by  the  fraction  T-i¥  which  is  represented  by  A.  The 
product  of  the  first  two  terms  gives  external  work  in  foot- 
pounds during  vaporization,  and  dividing  this  by  778  (Joule's 
Equivalent)  converts  it  to  heat  units  to  correspond  with  the 
other  values.  It  should  be  noted  that  P  in  this  expression 
stands  for  pressure  in  pounds  per  square  foot. 

The  total  latent  heat  of  vaporization  is  generally  desig- 
nated by  r  or  by  L,  and  it  follows  from  what  has  preceded 
that  j 

r  =  p+APu (13) 

The  value  of  r  for  atmospheric  pressure,  that  is,  for  a 
temperature  of  vaporization  of  212°  F.,  is  very  often  used 
in  engineering  and  should  be  memorized.  Its  value  is  now 
generally  taken  as  970.4  B.t.u.,  though  recent  work  would 
seem  to  indicate  a  value  of  about  972  as  nearer  the  truth. 

22.  Total  Heat  of  Dry  Saturated  Steam,  X  or  H.     The 
total  heat  required  to  convert  a  pound  of  water  at  32°  F. 
into  a  pound  of  saturated  vapor  at  some  temperature  tv  is 
called  the  total  heat  of  the  steam  or  the  heat  above  32°  and 
is  designated  by  X  or  by  H.     It  is  obviously  the  sum  of  the 
quantities  which  have  just  been  considered,  so  that 

\  =  q+r  =  q  +  p+APu.       ....      (14) 

23.  Total  Heat  of  Wet  Steam.     In  practical  work  the 
engineer  seldom  deals  with  pure  saturated  steam,  the  satu- 
rated vapor  nearly  always  carrying  in  suspension  more  or 
less  liquid  water  at  its  own  temperature.     To  distinguish 
between  saturated  steam  which  carries  liquid  water  and  that 
which  does  not,  the  former  is  called  wet  steam  or  wet 
saturated  steam,  and  the  latter  dry  saturated  steam. 

The  condition  of  dryness  or  wetness  is  described  by  what 
is  known  as  the  quality  of  the  steam.  Dry  saturated  steam 
is  said  to  have  a  quality  of  100  per  cent  while  saturated 
steam  carrying  10  per  cent  by  weight  of  liquid  is  said  to 


34  STEAM  POWER 

have  a  quality  of  90  per  cent.  Quality  expressed  as  a 
decimal  fraction  is  designated  by  the  letter  x,  so  that  if  x 
is  said  to  be  equal  to  0.8  in  referring  to  a  certain  sample  of 
steam,  it  means  that  that  steam  sample  consists  of  80  per 
cent  by  weight  of  saturated  steam  and  20  per  cent  liquid 
at  the  same  temperature. 

Since  the  water  in  wet  steam  has  the  same  temperature  as 
the  steam,  it  contains  all  the  heat  of  the  liquid  which  it  would 
contain  if  it  had  been  converted  into  steam,  but  it  obviously 
contains  no  latent  heat  of  vaporization.  It  follows  that  the 
total  heat  in  a  pound  of  wet  steam  (one  pound  of  a  mixture  of 
saturated  steam  and  water)  with  quality  equal  to  x  is 

Heat  per  pound  =  q  -f-  xr  =  q + xp + x  A  Pu  .     .     (15) 

The  letter  X  should  never  be  used  in  designating  the  total 
heat  per  pound  of  wet  steam,  as  it  has  been  chosen  as  the 
symbol  of  the  total  heat  per  pound  of  dry,  saturated  steam. 

24.  Heat  of  Superheat.  When  the  temperature  of 
saturated  steam  is  raised  by  the  addition  of  more  heat,  that 
is,  when  it  is  superheated,  a  very  definite  quantity  of  heat 
is  required.  The  quantity  required  per  pound  per  degivcc 
would,  by  definition,  be  the  specific  heat  of  the  material  in 
question. 

If  the  specific  heat  of  superheated  steam  were  reasonably 
constant,  the  heat  required  to  raise  its  temperature  at 
constant  pressure  from  saturation  temperature  to  some 
higher  value  fe  would  be  given  by  the  expression 

Heat  required  per  pound  =  Cp(t2  — 10) 

but  superheated  steam,  as  handled  by  the  engineer,  is 
generally  comparatively  near  the  saturated  condition,  and 
under  these  circumstances  the  values  of  the  specific  heat  vary 
rapidly  with  changes  of  pressure  and  temperature.  The 
extent  of  these  variations  is  shown  in  Fig.  13.  It  will  be 
observed  that  for  low  pressures  the  specific  heat  is  approxi- 


STEAM 


35 


mately  constant  at  a  value  below  0.5  for  any  given  pressure, 
but  that  for  very  high  pressures  it  varies  widely  over  a 
comparatively  small  temperature  range.  Thus  at  600  Ibs. 
per  square  inch  the  specific  heat  changes  from  unity  at 
about  510°  F.  to  0.6  at  about  550°  F. 

Practically,  it  is  customary  to  use  the  type  of  equation 
just  given  and  to  substitute  a  mean  specific  heat  over  the 
required  temperature  range  for  the  specific  heat  which  can- 
not be  assumed  constant  without  too  great  an  error.  The 


0          100         200         300         400         500 
Temperatures  Deg-.  Fahr. 

FIG.  13. — Progressive  Values  of  Specific  Heat,  Cp,  Water  Vapor. 

equation  for  heat  required  to  raise  the  temperature  from 
tv  to  t2  is  then 


Heat  of  superheat  per  pound  = 


(16) 


in  which  Cpm  stands  for  the  mean  specific  heat  at  constant 
pressure  over  the  temperature  range  from  tv  to  t%. 

Values  of  mean  specific  heats  of  superheated  steam  are 
given  in  Fig.  14,  the  values  indicated  by  the  curves  giving 
the  mean  specific  heat  between  saturation  temperatures  and 
Various  higher  temperatures  at  different  pressures. 


36 


STEAM  POWER 


25.  Total  Heat  of  Superheated  Steam.  The  total  heat 
required  to  convert  one  pound  of  water  at  32°  F.  into 
superheated  steam  at  a  temperature  of  fe°  F.  under  constant 
pressure  conditions  is  obviously 

Total  heat  per  pound  =  q+r+Cpm(t2 -t9).    .     (17) 


50 


100  150  200  250 

Temperatures  above  Saturation0!?. 


300 


FIG.  14. — Variation  of  Mean  Specific  Heat,  Water  Vapor. 

and  representing  the  degrees  of  superheat  (fe  —  tv)  by  D,  as 
is  customary,  this  becomes 

Total  heat  per  pound  =  q+r+CpmD.     .     .     (18) 

26.  Specific  Volume  of  Dry  Saturated  Steam,  V  or  S.   The 

volume  occupied  by  one  pound  of  a  substance  is  spoken  of  as 
the  specific  volume  of  that  material.  In  the  case  of  dry 
saturated  steam  there  are  as  many  specific  volumes  as  there 
are  pressures  under  which  the  steam  can  exist.  These 
values  are  generally  tabulated  in  steam  tables  and  are 
represented  by  the  letter  V  or  the  letter  S. 


STEAM 


37 


The  values  of  the  specific  volumes  of  steam  at  different 
pressures  are  given  in  Fig.  15.  It  is  important  to  note  the 
very  gradual  change  of  specific  volume  at  high  pressures 
and  the  very  rapid  change  and  enormous  increase  at  low 
pressures.  These  facts  have  considerable  influence  on  steam 
engineering  practice. 


0         24         0         8         10        12        14        16        18       20       22 
Specific  Volume  of  Dry  Saturated  Steaua  (Cubic  Feet) 

FIG.  15. — Pressure- Volume  Relations,  Saturated  Water  Vapor. 

A  curve  giving  properties  of  saturated  steam  is  called  a 
saturation  curve,  so  that  this  name  may  be,  and  often  is, 
applied  to  the  curve  given  in  Fig.  15. 

The  volume  occupied  at  any  pressure  by  half  a  pound 
of  dry  saturated  steam  will  obviously  be  half  that  occupied 
by  one  pound  of  such  material  at  the  same  pressure,  and 


38  STEAM  POWER 

the  same  statement  can  be  made  for  any  other  fraction  of 
a  pound.  It  follows  that  if  the  small  volume  occupied  by 
liquid  water  in  wet  steam  be  neglected,  the  volume  occupied 
by  one  pound  of  steam  (mixture)  of  50  per  cent  quality 
can  be  assumed  equal  to  half  that  occupied  by  an  equal 
weight  of  dry  saturated  steam  at  the  same  pressure.  A 
similar  statement  could  of  course  be  made  for  any  other 
quality  and  a  corresponding  fraction. 

Hence  if  one  pound  of  "  wet  steam  "  at  a  given  pressure 
is  found  to  have  such  a  volume  that  it  would  be  indicated  by 
point  b  in  Fig.  16,  the  quality  of  this  material  must  be  given 

by  the   expression  x  =  -—  if  the    volume   occupied    by   the 

liquid  water  in  the  mixture  be 
neglected. 

27.  Specific   Density  of  Dry 

Saturated  Steam,  -  or  5.     The 

weight  per  cubic  foot  of  saturated 
steam  is  spoken  of  as  its  specific 
density .:  The  specific  density 
is  obviously  the  reciprocal  of 

the'  specific  volume  and  is  there- 
FIG.  16. — Determining  Quality 

from  Volume.  fore  — 

28.  Reversal  of  the  Phenomena  Just  Described.     If  any 

process  which  has  resulted  in  the  absorption  of  a  quantity 
of  heat  by  a  substance  be  carried  through  in  the  reverse 
direction,  the  same  amount  of  heat  will  again  be  given  up. 
It  follows  that  a  pound  of  dry  saturated  steam  will  give  up 
the  total  latent  heat  of  vaporization  when  condensed  to 
liquid  at  the  same  temperature,  and  that  the  resultant  pound 
of  hot  water  will  give  up  the  total  heat  of  the  liquid  if  cooled 
to  32°  F. 

29.  Generation  of  Steam  in  Real  Steam  Boiler.     The 
steam  boiler  is  equivalent  to  a  vessel  partly  filled  with  water 


STEAM 


39 


and  fitted  with  means  for  supplying  heat  to  the  water  and 
for  carrying  off  the  vapor  formed.  This  is  shown  diagram- 
matically  in  Fig.  17.  At  first  glance  this  would  not  seem 
to  be  at  all  similar  to  the  cylinder  and  piston  already  con- 
sidered, but  it  really  is  the  exact  equivalent  so  far  as  the 
generation  of  steam  is  concerned.  The  flow  of  steam  out 
of  the  steam-pipe  is  restricted  to  the  extent  necessary  to 
maintain  a  high  and  constant  pressure  within  the  boiler,  and, 
when  in  regular  operation,  steam  is  formed  within  the 


o  r  -     :q  a 


1" 
.               •     "•„  "           "  v>      "       ••    «•'     '  o       ..  '     •  ,  - 

r*  "  "  "'/* 

fe 

1  

$ 

r-  _-_-_—  _^-^_- 

l)'w/i 
///'•v 

jMiiiJi£^' 

t=r-^—  —  —    _    _—  -l-pr-pr} 

FIG.  17. — Formation  of  Steam  in  a  Steam  Boiler. 

boiler  under  this  pressure  just  as  fast  as  necessary  to  replace 
that  flowing  out. 

By  picturing  the  steam  as  flowing  out  in  layers  or  lamina 
these  lamina  can  be  imagined  as  taking  the  place  of  the 
piston  in  the  apparatus  of  Fig.  11,  and  each  pound  of  steam 
formed  will  then  push  a  piston  before  it  exactly  as  was 
assumed  in  the  previous  discussion. 

30.  Gauge  Pressure.  The  steam  pressure  in  a  boiler  is 
commonly  determined  by  means  of  an  instrument  called  a 
pressure  gauge.  These  instruments  are  almost  always  con- 
structed about  as  shown  in  Fig.  18  (a)  and  (6) .  The  Bourdon 
spring  is  a  tube  of  elliptical  section  bent  approximately  into 


40 


STEAM  POWER 


the  arc  of  a  circle.  One  end  of  this  tube  is  connected  directly 
to  the  pressure  connection  of  the  gauge  and  the  other  end 
is  closed  and  connected  to  a  toothed  sector  as  shown. 

When  the  pressure  inside  a  tube  of  this  character  is 
increased,  the  tube  has  a  tendency  to  unroll  or  straighten 
out,  and  in  so  doing  it  moves  the  toothed  sector  in  such  a 
way  as  to  rotate  the  pointer  or  gauge  hand  and  make  its  end 
move  over  the  scale  in  the  direction  of  increasing  pressure. 
With  diminishing  pressure  the  tube  again  rolls  up  and 
rotates  the  hand  in  the  opposite  direction. 


(a)  (6) 

FIG.  18. — Bourdon  Pressure  Gauge. 

Instruments  of  this  kind  are  so  made  and  adjusted  that 
the  hand  points  to  zero  when  the  gauge  is  left  open  to 
the  atmosphere.  Under  such  conditions  the  pressure  inside 
the  tube  is  equal  to  that  of  the  atmosphere  and  is  not  zero. 
The  gauge  therefore  only  indicates  pressures  above  atmos- 
pheric on  its  scale,  and  the  total  pressure  inside  the  boiler 
is  really  that  shown  by  the  gauge  plus  that  of  the  atmos- 
phere. 

Pressures  as  indicated  by  the  gauge  are  called  gauge 
pressures.  Pressures  obtained  by  adding  the  pressure  of 


STEAM  41 

the  atmosphere  to  the  reading  of  the  gauge  are  known  as 
absolute  pressures.     Then 

* 

Absolute  Pressure  =  Gauge  Pressure-}- Atmospheric  Pressure 
and 

Gauge  Pressure  =  Absolute  Pressure  — Atmospheric  Pressure. 

In  accurate  work  the  existing  atmospheric  pressure 
should  be  determined  by  means  of  the  barometer,  but  for 
ordinary,  approximate  calculations  and  for  cases  in  which 
no  barometric  data  are  available,  it  is  customary  to  assume 
the  pressure  of  the  atmosphere  to  be  equal  to  14.7  Ibs.  per 
square  inch.  This  is  very  nearly  true,  on  the  average,  at 
sea  level,  but  is  generally  far  from  true  at  higher  elevations. 

PROBLEMS 

1.  Determine  by  means  of  the  steam  tables  the  temperatures, 
total  heats,  heats  of  liquid,  internal  and  external  latent  heats,  and 
the  specific  volumes  of  1  Ib.  of  dry,  saturated  steam  under  the  fol- 
lowing absolute  pressures  (Ibs.  per  sq.  in.):  15,  50,  95,  180  and  400. 

2.  Determine  the  heats  of  the  liquid,  latent  heats  of  vapor- 
ization and  total  heats  for  2  Ibs.  of  dry  saturated  steam  at  the 
following  temperatures  in  °F.:   101.83,  212  and  327.8. 

3.  Determine  the  volumes  occupied  by  2  Ibs.  of  dry  saturated 
steam  under  the  conditions  of  problem  2. 

4.  Determine  the  heats  of  the  liquid,  latent  heats  of  vapor- 
ization and  total  heats  for  1  Ib.  of  saturated  steam  with  a  quality 
of  90%  at  the  following  absolute  pressures:  25,  50,  75,  125. 

5.  Determine  the  total  heat  above  32°  F.  in  12  Ibs.  of  saturated 
steam  with  quality  of  97%  at  a  pressure  of  125  Ibs.  per  square  inch 
absolute. 

6.  AVhat  space  will  be  filled  by  20  Ibs.  of  dry  saturated  steam 
at  a  pressure  of  150  Ibs.  per  square  inch  absolute? 

7.  What  space  will  be  filled  by  20  Ibs.  of  saturated  steam  at  a 
pressure  of  150  Ibs.  per  square  inch  absolute  and  with  a  quality 
of  95%  if  the  volume  occupied  by  the  water  present  be  neglected? 

8.  How  many  pounds  of  dry  saturated  steam  at  a  pressure  of 
75  Ibs.  per  square  inch  absolute  will  be  required  to  fill  a  space  of 
10  cu.  ft.? 


42  STEAM  POWER 

X  9.  How  many  pounds  of  saturated  steam  with  quality  96% 
and  at  a  pressure  of  1 10  Ibs.  per  square  inch  absolute  will  be  required 
to  fill  a  space  of  8  cu.  ft.? 

10.  How  much  external  work,  measured  in  B.t.u.,  is  done  when 
1  Ib.  of  water  at  the  temperature  of  212°  F.  is  converted  into  dry 
saturated  vapor  at  the  same  temperature? 

X  11.  How  much  external  work,  measured  in  foot-pounds,  is 
done  when  2  Ibs.  of  water  at  a  temperature  of  212°  F.  are  converted 
into  90%  quality  steam  at  the  same  temperature? 

12.  How  much  heat  is  required  for  doing  internal  work  during 
the  vaporization  of  1  Ib.  of  water  under  such  conditions  that  the 
total  latent  heat  of  vaporization  is  852.7  B.t.u.  and  the  external 
latent  heat  is  83.3  B.t.u.? 

13.  What  is  the  quality  of  steam  containing  1000  B.t.u.  above 
32°  F.  per  pound  when  under  a  pressure  of  150  Ibs.  per  square 
inch  absolute? 

)(  14.  Heat  is  added  to  1  Ib.  of  mixed  steam  and  water  while  the 
pressure  is  maintained  constant  at  100  Ibs.  per  square  inch  absolute. 
The  percentage  of  steam  in  the  mixture  is  increased  thereby  from 
50%  to  95%. 

(a)  How  much  heat  was  added? 

(b)  How  much  internal  latent  heat  was  added? 

(c)  How  much  external  latent  heat  was  added? 

15.  How  much  heat  is  required  to  completely  vaporize  1000 
Ibs.  of  water  at  a  temperature  of  92°  F.  when  pumped  into  a  boiler 
in  which  steam  is  generated  at  a  pressure  of  150  Ibs.  per  square 
inch  gauge?     Note  that  heat  above  32°  F.  in  92°  F.  water  is  given 
as  q  in  steam  tables  for  a  temperature  of  92°  F.  //#j~-  ^  '  /J^S 

16.  Find  the  amount  of  heat  necessary  to  produce  in  a  boiler 
200  Ibs.  of  steam  having  a  quality  of  97%  at  a  pressure  of  100  Ibs. 
gauge  when  the  feed  water  has  a  temperature  of  205°  F. 

17.  What  volume  would  be  occupied  by  the  material  leaving 
the  boiler  in  problem  16,  neglecting  volume  occupied  by  water? 


CHAPTER   IV 
THE   IDEAL   STEAM   ENGINE 

31.  The  Engine.  If  the  cylinder  and  piston  assumed  in 
the  discussion*  of  the  last  chapter  be  imagined  as  turned 
into  a  horizontal  position  and  fitted  with  a  frame,  piston 


Fly  wheel 


FIG.  19. — Sirr.ple  Steam  Engine. 

rod,  crosshead,  connecting  rod,  crank  shaft  and  flywheel 
as  in  Fig.  19,  a  device  results  which  might  be  used  as  a 
steam  engine  for  the  production  of  power.  By  adding  heat 
to,  and  taking  heat  from,  the  water  and  steam  in  the  cylin- 
der in  the  proper  way  and  at  the  proper  time,  the  water 
and  steam,  or  working  substance,  can  be  made  to  do  work 
upon  the  piston.  The  piston  can  transmit  this  work 
through  the  mechanism  to  the  rim  of  the  flywheel,  and  it 
can  be  taken  from  the  rim  by  a  belt  connected  to  a  pulley 
on  a  machine  which  is  to  be  driven. 

43 


44 


STEAM  POWER 


To  make  the  analysis  easier,  a  simplified  type  of  engine 
will  be  assumed.  It  is  shown  in  Fig.  20  and  consists  of  the 
same  cylinder,  piston  and  piston  rod  as  just  described. 
A  wire  is  fastened  to  the  end  of  the  piston  rod  and  run  back 
over  a  pulley  in  such  a  way  that  a  weight  fastened  to  the 
free  end  of  the  wire  will  be  raised  if  the  piston  moves  out. 
The  weight  is  made  up  of  two  parts,  one  large  and  one  small. 
When  both  are  on  the  wire  the  pull  which  they  exert  causes 
the  piston  to  exert  a  high  pressure  upon  whatever  is  con- 


Cy'linder! 


Volume 


FIG.  20. — Simplified  Steam  Engine. 

tained  in  the  cylinder.  When  only  the  small  weight  hangs 
on  the  wire,  the  piston  exerts  a  much  lower  pressure  upon 
the  material  in  the  cylinder. 

Imagine  that  the  piston  and  the  walls  of  the  cylinder 
are  made  of  some  ideal  material  which  will  not  receive  or 
conduct  heat.  Imagine  also  that  the  cylinder  is  fitted 
with  a  permanent  head  which  is  a  perfect  conductor  of 
heat.  These  conditions  are  of  course  ideal  but  are  assumed 
for  the  sake  of  simplicity. 

Assume  further  that,  when  one  pound  of  water  is  con- 


THE  IDEAL  STEAM  ENGINE  45 

tained  in  the  cylinder  and  the  piston  is  driven  into  the 
cylinder  by  the  two  weights  until  the  space  between  the 
piston  and  the  cylinder  head  is  just  large  enough  to 
contain  the  pound  of  water,  the  piston  exerts  a  high 
pressure  equal  to  PI  pounds  per  square  foot  against  the 
water.  The  volume  of  this  water  and  the  pressure  upon 
it  can  be  represented  by  the  point  a  of  the  PV  diagram, 
Fig.  20. 

32.  Operation  of  the  Engine.  With  conditions  as  de- 
scribed in  the  preceding  paragraphs,  imagine  a  flame  or 
other  source  of  heat  at  high  temperature  to  be  brought  into 
contact  with  the  conducting  cylinder  head  and  to  pass  heat 
into  the  cylinder,  raise  the  temperature  of  the  water  within 
to  the  temperature  of  vaporization  and  ultimately  vaporize 
it.  As  the  water  vaporizes  it  will  push  the  piston  out  of  the 
cylinder  just  as  described  in  the  last  chapter  and  a  hori- 
zontal line  such  as  ab  in  Fig.  20  will  represent  the  increase 
of  volume  (vaporization)  at  constant  pressure.  The  point 
b  may  be  assumed  to  represent  the  volume  of  one  pound 
of  dry  saturated  vapor  at  a  pressure  PI.  Obviously  the 
steam,  as  it  is  formed,  does  work  in  driving  out  the  piston 
against  the  resistance  offered  by  the  weights  which  must  be 
raised. 

If  a  stop  is  provided  which  will  prevent  the  movement 
of  the  piston  beyond  the  position  corresponding  to  the 
point  6,  it  will  be  possible  to  remove  the  larger  weight  when 
that  point  is  reached  and  the  high  pressure  steam  will  hold 
the  piston  and  rod  hard  against  the  stop.  If  now  some 
cooling  medium  is  applied,  such  as  a  large  piece  of  ice  held 
against  the  conducting  head  of  the  cylinder  or  water  running 
over  that  head,  heat  will  be  abstracted  and  a  partial  con- 
densation of  the  steam  within  the  cylinder  will  occur. 
As  condensation  progresses  the  pressure  will  drop  because 
there  will  be  less  and  less  steam,  by  weight,  in  a  given 
volume.  Such  a  process,  would  be  indicated  by  the  line 
be  which  represents  a  drop  of  pressure,  while  the  volume 


46  STEAM  POWER 

contained  within  the  cylinder  walls  between  head  and 
piston  remains  constant. 

When  some  point  c  is  reached,  the  steam  pressure  will 
have  been  reduced  to  a  value  equal  to  that  exerted  by  the 
small  weight,  and  the  piston  will  be  driven  in  toward  the 
cylinder  head  while  the  heat  absorbing  medium  continues 
to  remove  heat  from  the  steam  and  to  cause  further  conden- 
sation. The  combination  of  piston  motion  and  heat  ab- 
sorption will  be  so  regulated  that  the  pressure  remains  con- 
stant at  P2  during  this  process,  because  the  weight  will 
move  the  piston  inward  just  as  fast  as  necessary  to  main- 
tain a  constant  pressure.  If  sufficient  heat  is  absorbed, 
the  pound  of  material  within  the  cylinder  will  ultimately 
all  be  condensed  or  liquefied  and  will  just  fill  the  volume  Vd. 

The  heat  absorbing  body  may  now  be  removed  and  an 
infinitesimal  motion  of  the  piston  toward  the  head  would 
serve  to  raise  the  pressure  on  the  liquid  water  from  P%  to 
PI  so  that  the  volume  Vd  may  be  taken  equal  to  the  volume 
Va  and  the  line  da  may  be  assumed  to  be  vertical.  It 
would  then  represent'  an  increase  of  pressure  at  constant 
volume.  This  might  be  caused  by  hanging  a  weight  of  the 
larger  size  on  the  wire  when  condition  d  was  reached. 

Having  brought  the  material,  or  working  substance, 
back  to  the  conditions  originally  shown  at  a,  the  high 
temperature  source  of  heat  can  again  be  brought  in  contact 
with  the  end  of  the  cylinder  and  the  entire  cycle  carried 
through  once  more.  There  is  obviously  no  reason  why  it 
could  not  be  repeated  as  often  as  desired. 

33.  Work  Done  by  the  Engine.  If  the  device  just 
described  is  to  serve  as  a  steam  engine,  it  must  actually 
make  mechanical  energy  available,  that  is,  it  must  convert 
into  mechanical  form,  some  of  the  heat  energy  supplied  it. 
It  is  now  necessary  to  see  whether  it  does  so. 

Water  vaporizing  and  increasing  in  volume  as  from 
Va  to  F&  was  shown  in  the  last  chapter  to  do  work  upon  the 
piston  confining  it.  Work  has  been  shown  to  be  equal  to 


THE  IDEAL  STEAM  ENGINE  47 

(total  force  X  total  distance)  and  in  this  case  if  L  repre- 
sents the  distance"  in  feet  traveled  by  the  piston,  the  work 
done  by  the  steam  upon  the  piston  while  the  latter  moves 
from  a'  to  b'  must  be 

Work  done  on  piston  =  total  force  X  distance    , 

=  PI  X area  of  piston  XL  .  .  .  ft.-lbs. 

But  the  product  of  area  of  piston  in  square  feet  by 
distance  traveled  in  feet  is  equal  to  the  piston  displace- 
ment or  volume  swept  through  by  the  piston,  that  is 
(Vb—  Va)  cubic  feet.  Therefore 

Work  done  on  piston  =  Pi(Vb-  Va)  ft.-lbs.   .     (19a) 

Pl(Vb-Va) 


778 


B.t.u.    .     (196) 


The  first  form  of  this  expression  Pi(F&—  Va)  is  very 
obviously  represented  by  the  area  under  the  line  ab  in 
Fig.  20  and  this  area  therefore  represents  the  work  done  by 
the  steam  upon  the  piston  during  the  change  of  volume  at 
constant  pressure  represented  by  that  line.  While  the 
steam  is  supplying  this  amount  of  energy  to  the  piston  or 
doing  this  amount  of  work  upon  the  piston,  the  latter  does 
an  equivalent  amount  of  work  upon  the  weights  if  friction- 
less  mechanism  be  assumed.  In  such  a  case  the  total 
weight  hung  on  the  wire  multiplied  by  the  distance  raised 
would  therefore  give  the  same  result  in  foot-pounds  as 
that  just  obtained. 

It  should  be  noted  that  Eq.  (19)  is  merely  an  expression 
of  the  external  work  done  during  vaporization,  that  is, 
.•in  expression  of  the  amount  of  heat  which  is  used  for  the 
doing  of  external  work.  It  is  the  exact  equivalent  of  the 
external  latent  heat  previously  discussed.  In  fact,'  the 
group  of  symbols  APu  is  really  a  condensation  of  Eq.  (19) 

formed  by  putting  A  for  — —  and  u  for  (F&—  Fa). 

7/8 


48  STEAM  POWER 

The  line  cd  also  represents  a  change  of  volume  at  con- 
stant pressure  and  the  same  type  of  formula  as  applied 
to  ab  will  express  the  work  done  during  this  process.  In 
this  case,  however,  the  piston  is  being  pushed  into  the 
cylinder  by  the  small  weight  against  the  pressure  of  the 
steam,  and  energy  is  being  supplied  to  push  the  piston  in. 
This  energy  is  equal  to  the  weight  of  the  small  weight 
(pounds)  multiplied  by  the  distance  it  falls  (feet).  The 
piston  is  therefore  doing  work  upon  the  steam,  and  the 
amount  is 

Work  done  on  steam  =  P2(Fc—Fd)  ft, -Ibs.    .     .     (20) 
-^=^>B.t.u.     .     .     (21, 

The  first  form  of  expression  also  represents  the  area  under 
the  line  cd  and  this  area  therefore  represents  the  work 
done  by  the  piston  upon  the  steam  mixture  in  the  cylinder 
during  the  process  represented  by  cd. 

No  work  can  be  done  by  steam  on  piston  or  by  pistor 
on  steam  during  the  processes  represented  by  be  or  da 
because  both  the  weights  and  the  piston  are  stationan* 
during  these  changes  and  it  has  already  been  shown  that 
work  involves  motion. 

The  total  work  done  upon  the  piston  by  the  steam 
is  therefore  represented  by  the  area  abef  and  this  amount 
of  energy  is  used  in  raising  the  two  weights  through  a 
vertical  distance  equal  to  the  piston  travel.  Some  of 
this  energy,  or  its  equivalent,  will  have  to  be  returned  an 
instant  later,  however,  in  order  that  the  piston  may  do  the 
work  shown  by  the  area  cdfe  upon  the  steam.  It  is  returned 
by  the  small  weight  dropping  through  a  distance  equal  to 
the  travel  of  the  piston.  The  net  mechanical  energy 
made  available  by  carrying  through  the  series  of  processes 
is  therefore  represented  by  the  area  (abef)  —  (cdfe)  =  (abed} 
or  the  area  enclosed  by  the  four  lines  representing  the 


THE  IDEAL  STEAM  ENGINE  49 

pressure  and  volume  changes  experienced  by  the  working 
substance  during  one  cycle  of  events.  It  is  equal  to  the 
work  done  in  raising  the  larger  weight  a  vertical  distance 
equal  to  the  travel  of  the  piston. 

This  net  energy  made  available  is  obviously 

Energy  made  available  =  PI (Vb-  Va)  -P2(Ve-  Va} 

=  (Pi - P2)(Vb-  Va)  ft.-lbs.    (22) 

(Pl-P2}(Vb-Va) 


778 


B.t.u.    (23) 


Since  this  amount  of  energy  is  made  available  while  one 
cycle  of  events  is  being  carried  out  and  since  the  cycle 
can  be  repeated  time  after  time  if  sufficient  heating  and 
cooling  mediums  are  available,  any  quantity  of  mechanical 
energy  can  be  produced  from  heat  energy  by  repeating  the 
cycle  a  sufficient  number  of  times.  This  would  correspond 
to  picking  up  a  number  of  the  larger  weights  which  were 
slid  on  to  the  wire  at  the  lower  elevation  and  slid  off  at 
the  higher. 

This  repetition  of  cycles  would  correspond,  in  a  real 
engine,  to  running  at  such  a  speed  that  the  required  number 
of  cycles  would  be  produced  in  a  given  time  to  make  avail- 
able the  amount  of  mechanical  energy  required. 

Or,  the  power  made  available  per  cycle  could  be  increased. 
This  is  easily  seen  by  an  inspection  of  Eq.  (22).  Increas- 
ing the  value  of  either  of  the  right-hand  terms  will  obviously 
increase  the  amount  of  energy  made  available.  The 
value  of  (Pi  —  Pz)  can  be  increased  by  raising  the  initial 
pressure  PI  or  by  lowering  the  final  pressure  PI.  The  value 
of  (F&—  Fa)  may  be  increased  by  using  more  than  one  pound 
of  material,  thus  increasing  both  the  volume  Vb  of  the  satu- 
rated steam  formed  and  increasing  the  volume  Va  of  the 
liquid  water,  but  getting  a  greater  numerical  value  for 
(Vb—  Va).  This  would  correspond  in  a  real  case  to  using 
a  larger  cylinder  and  therefore  a  larger  engine. 


50  STEAM  POWER 

ILLUSTRATIVE   PROBLEM 

An  engine  of  the  type  described  is  to  work  with  a  maximum 
pressure  of  100  Ibs.  per  square  inch  absolute  and  a  minimum 
pressure  of  15  Ibs.  per  square  inch  absolute.  The  cylinder  is  to 
be  of  such  size  that  1  Ib.  of  water  is  used  and  the  steam  is  to  be 
dry  and  saturated  at  the  point  b  of  the  cycle. 

Find:  (a)  the  amount  of  mechanical  energy  made  available 
per  cycle;  (b)  the  amount  of  energy  made  available  per  minute 
if  150  cycles  are  produced  per  minute;  and  (c)  the  horse  power  of 
the  engine. 

It  will  first  be  necessary  to  find  the  piston  displacement  required 
and  the  space  necessary  between  piston  and  cylinder  head  to 
accommodate  the  pound  of  water  in  liquid  form.  The  steam  tables 
give  the  volume  of  one  pound  of  dry  saturated  steam  at  100  Ibs. 
per  square  inch  as  4.429  cu.ft.  and  the  volume  of  one  pound  of 
water  may  be  taken  as  0.017  cu.ft.  The  values  of  the  various 
volumes  and  pressures  will  therefore  be 

Va  =  Fd  =0.017  cu.ft.; 

Vb  =  Vc  =4.429  cu.ft.; 

pa  =pb  =  100 X 144  =  14,400  Ibs.  per  sq.ft.; 

Pc=Pd  =  15X144  =2160  Ibs.  per  sq.ft. 

(a)  Using  Eq.  (22)  the  amount  of  mechanical  energy  made 
available  per  cycle  will  be 

(P,  -TV) (Vb  -  Va)  =  (14,400  -2160) (4.429  -0.017) 
=  12,240X4.412; 
=  54,002.88  Tt.lbs. 

(6)  If  150  cycles  are  produced  per  minute,  the  total  amount 
of  mechanical  energy  made  available  per  minute  must  be 

150X54,002.88-8,100,300  ft.-lbs. 
(c)  The  horse  power  must  then  be 

8,100,300 
h*-- -33^00- 

34.  Heat  Quantities  Involved.  It  is  a  very  simple 
matter  to  determine  the  quantity  of  heat  which  must  be 
supplied  to  produce  the  process  abt  and  the  quantities  of 


THE  IDEAL  STEAM  ENGINE  51 

heat  which  must  be  removed  to  produce  the  processes 
be  and  cd.  This  can  be  done  by  making  use  of  the  known 
properties  of  water  and  steam  as  given  in  the  steam 
tables. 

The  water  at  d  must  be  at  the  temperature  of  vaporiza- 
tion corresponding  to  pressure  PI  since  it  has  just  been 
formed  by  condensation  from  steam  under  that  pressure. 
It  therefore  contains  the  heat  of  the  liquid  corresponding 
to  that  pressure.  If  it  is  to  be  vaporized  at  pressure  PI,  it 
must  first  be  raised  to  the  higher  temperature  corresponding 
to  that  pressure.  The  amount  of  heat  required  to  do  this 
will  obviously  be  the  difference  between  the  heat  of  the 
liquid  at  the  temperature  corresponding  to  PI  and  the  heat 
of  the  liquid  at  the  temperature  corresponding  to  P^ 
These  can  be  found  in  the  steam  tables. 

The  latent  heat  of  vaporization  at  PI  must  then  be  added 
to  cause  the  increase  of  volume  shown  by  ab.  This  can 
also  be  found  in  the  steam  tables  for  any  given  case. 

The  quantity  of  heat  which  must  be  removed  to  produce 
the  processes  represented  by  be  and  cd  can  be  found  sim- 
ilarly from  steam  table  values,  although  the  exact  method 
of  procedure  is  not  quite  as  obvious  as  in  the  preceding 
cases. 

Assuming  that  it  is  possible  to  find  the  heat  supplied, 
Qi,  and  the  heat  removed,  $2,  it  is  obvious  that  the  energy 
made  available  in  mechanical  form,  per  cycle,  must  be 
equal  to  (Qi  —  Qz)  B.t.u.,  since  this  is  the  amount  of  heat 
energy  which  has  disappeared  and  since  it  cannot  have 
been  destroyed.  This  may  be  put  in  the  form  of  an  equa- 
tion, thus 

Energy  made  available  =  Qi  —  Q.2.       .     .     (24) 

If  the  proper  substitutions  are  made  in  this  formula  and  it 
is  then  simplified,  it  becomes 

Energy  made  available  =;(APu)Pi—xc(APu)Ps  B.t.u.,    (25) 


52  STEAM  POWER 

in  which 

p^ihe  external  latent  heat  at  pressure  PI; 
p2  =  the:    external    latent   heat   at   pressure  P%,   and 
xc  =  quality  at  point  c,   which   can  be    found    from 
the  ratio  of  dc  to  dc'\ 

Numerical  substitution  in  this  equation  for  any  given 
case  will  show  that  it  gives  exactly  the  same  values  as  would 
be  obtained  by  the  use  of  Eq.  (23). 

It  is  to  be  noted  particularly  that  the  energy  made 
available  is  actually  less  than  the  external  latent  heat  at 
the  higher  pressure,  while  the  heat  supplied  must  be  equal 
to  the  total  latent  heat  plus  some  of  the  heat  of  the  liquid. 
An  inspection  of  the  steam  tables  will  show  that  the  exter- 
nal latent  heat  for  ordinary  steam  pressures  forms  a  very 
small  fraction  of  even  the  total  latent  heat,  and  therefore 
the  mechanical  energy  made  available  for  a  given  expendi- 
ture of  heat  energy  is  very  small  in  the  case  under  dis- 
cussion. 

35.  Efficiency.  The  term  efficiency  is  used  in  engineer- 
ing as  a  measure  of  the  return  obtained  for  a  given  expendi- 
ture. It  may  be  defined  in  any  one  of  the  following  ways: 

Useful  result 
Efficiency  = 


Expenditure  made  to  obtain  that  result 

Result 
Effort 

'  .......  (26) 


In  the  case  of  a  heat  engine,  the  useful  result  is  the 
mechanical  energy  obtained  by  the  operation  of  the  engine, 
while  the  expenditure  made  is  the  heat  which  is  supplied. 
For  this  case  efficiency  may  therefore  be  defined  by  the 
expression 


THE  IDEAL  STEAM  ENGINE  53 

Mechanical  energy  obtained  per  cycle 
Engine  efficiency  =  —  ,. 

Heat  supplied  per  cycle 


=        ..........   «> 

in  which 

E  stands  for  mechanical  energy  obtained, 
Qi  stands  for  heat  supplied,  and 
Qz  stands  for  heat  rejected. 

In  the  case  of  the  type  of  steam  engine  just  considered, 
this  efficiency  would  have  a  value  between  6  and  8  per 
cent  for  ordinary  pressures.  That  is,  the  engine  would 
produce  in  mechanical  form  only  6  to  8  per  cent  of  the 
energy  supplied  it  in  the  form  of  high  temperature  heat. 
Moreover,  these  figures  would  hold  only  for  a  theoretically 
perfect  engine;  a  real  engine  built  to  operate  upon  this 
cycle  would  probably  give  efficiencies  of  the  order  of  2  to 
3  per  cent.  The  reasons  for  this  great  discrepancy  will  be 
discussed  in  a  later  chapter. 

36.  Effect  of  Wet  Steam.  In  what  has  preceded,  it 
was  assumed  that  the  pound  of  steam  was  completely  vapor- 
ized along  the  line  ab  so  that  dry,  saturated  steam  existed 
in  the  cylinder  at  b.  It  might,  however,  be  assumed  that 
vaporization  was  incomplete  at  the  upper  right-hand 
corner  of  the  cycle,  so  that  this  point  occurred  at  a  point 
to  the  left  of  6  and  with  a  quality  x  at  that  point  less  than 
unity. 

Under  such  conditions,  the  cylinder  would  not  have  to 
be  so  big,  since  the  maximum  volume  attained  by  the  steam 
would  be  smaller  than  in  the  preceding  case.  The  work 
done  per  cycle  would  obviously  be  smaller  in  quantity, 
because  the  area  enclosed  within  the  lines  of  the  cycle  would 
be  smaller.  It  can  also  be  shown  that  the  efficiency  would 
also  be  lowered  by  lowering  the  quality  at  b. 


54  STEAM  POWER 

37.  Application  to  a  Real  Engine.  The  engine  which 
has  been  described  in  the  preceding  paragraphs  could  easily 
be  converted  into  the  counterpart  of  a  real  engine  by  sub- 
stituting connecting  rod,  crank  shaft  and  flywheel  for  wire, 
pulley  and  weights  as  described  in  the  first  paragraph  of 
this  chapter.  It  could  then  be  made  to  do  work  in  just 
the  same  way  as  has  been  described;  some  of  the  energy 
made  available  during  the  outstroke  would  be  used  for 
overcoming  resistance  at  the  shaft,  that  is,  doing  useful  work, 
and  some  of  it  would  be  stored  in  the  flywheel  which  would 
speed  up  slightly.  The  energy  which  must  be  expended  on 
the  steam  during  the  return  stroke  would  be  obtained  by 
allowing  the  flywheel  to  slow  down  and  thus  deliver  suf- 
ficient kinetic  energy  to  drive  the  piston  back  against  the 
low-pressure  steam.  The  cycle  and  the  efficiency  would 
thus,  theoretically,  be  exactly  the  same  as  those  just  in- 
vestigated . 

Great  difficulty  would,  however,  be  met  in  a  real  engine 
if  the  steam  had  to  be  formed  and  condensed  within  the 
cylinder,  and  another  method  which  gives  the  same  results 
is  therefore  used.  Steam  is  generated  in  a  boiler  and 
allowed  to  flow  into  the  cylinder  and  push  out  the  piston 
just  as  though  it  were  actually  being  formed  in  the  cylin- 
der as  previously  described.  When  the  piston  reaches  the 
end  of  its  outstroke  the  inlet  valve  is  closed  and  the  exhaust 
valve  is  opened,  allowing  some  of  the  steam  to  blow  out 
into  a  space  in  which  a  lower  pressure  exists.  As  the 
piston  stands  still  at  the  end  of  its  stroke  while  the  pres- 
sure drops,  the  line  be  is  produced  as  in  the  previous  descrip- 
tion, but  by  a  different  method.  The  piston  then  returns 
and  drives  the  remaining  steam  out  of  the  cylinder  at  a 
constant  pressure  theoretically  equal  to  that  of  the  space 
into  which  the  steam  is  being  forced  or  exhausted.  The 
line  cd  is  thus  produced  and  the  closure  of  the  exhaust 
valve  and  opening  of  the  admission  valve  when  d  is  reached 
will  start  the  cycle  over  again. 


THE  IDEAL  STEAM  ENGINE  55 

In  order  to  get  more  work  out  of  a  given  size  of  cylinder 
and  to  obviate  the  necessity  of  giving  back  energy  which 
has  already  been  given  out,  engines  are  generally  made  to 
take  steam  on  both  sides  of  the  piston.  They  are  then 
known  as  double  acting  engines.  In  this  case  the  steam 
admitted  on  one  side  of  the  piston  would  supply  the  energy 
necessary  both  for  overcoming  the  resistance  due  to  the 
load  and  for  driving  out  the  low-pressure  steam  on  the 
other  side  of  the  piston.  On  the  return  stroke  conditions 
would  be  just  reversed. 

38.  Desirability  of  Other  Cycles.     The  cycle  of  opera- 
tions described  in  preceding  paragraphs  is  the  most  inef- 
ficient of  all  those  actually  used,  that  is,  it  gives  the  small- 
est  return   for   a   given   amount   of   heat   supplied.     This 
is  because  only  the  external  latent  heat  supplied  is  con- 
verted  into   mechanical   energy   and   part   of   that  energy 
must  be  returned  to  complete  the  cycle.     All  of  the  internal 
latent  heat  and  all  of  the  heat  of  the  liquid  supplied  along 
ab  pass  through   the  engine   without   conversion  and   are 
exhausted. 

Therefore,  cycles  which  differ  from  that  described 
in  such  a  way  as  to  make  it  possible  to  convert  into  mechani- 
cal energy  some  of  the  internal  latent  heat  and  possibly 
some  of  the  heat  of  the  liquid  should  be  highly  desirable 
as  they  ought  to  yield  a  larger  return  of  mechanical  energy 
for  the  same  total  amount  of  heat  supplied.  Two  such 
cycles  are  commonly  used;  they  may  be  described  as  the 
Complete-expansion  cycle  and  the  Incomplete-expansion  cycle. 
The  former  is  used  in  steam  turbines,  the  latter  in  most 
reciprocating  steam  engines.  The  rectangular  cycle  which 
has  just  been  described  is  used  in  duplex  pumps  and  similar 
apparatus. 

39.  The  Complete-expansion  Cycle.     This  cycle,  which 
is  also  known  as  the  Clausius  and  as  the  Rankine  cycle, 
starts  just  the  same  as  that  already  described.     This  is 
shown  in  Fig.  21.     The  pressure  on,  say,  a  pound  of  water 


56 


STEAM  POWER 


is  raised  from  P<z  to  PI  and  its  temperature  is  raised  from 
that  of  vaporization  at  P%  to  that  of  vaporization  at  PI. 
After  this  it  is  vaporized,  giving  the  increase  of  volume 


>Iled  Spring 

b 

1                                                              '      \C 

.  J   . 

1  \          /*\         '  •       i 

WW) 

\      /    »        '    N       /    '•      ' 

v'                Vv'                  'v                    v'  ' 

Saturation  Curve 


,Work  done  by  Steam 
.during  expansio    6-<? 


Volume 
FIG.  21. — Complete  Expansion  Cycle  or  Clausi^y  Cycle. 

shown  by  ab.  The  supply  of  heat  is  then  stopped.  The 
cylinder  of  the  engine  is  made  larger  than  in  the  preceding 
type  so  that  when  the  point  6'  is  reached  the  piston  can 
travel  still  further,  and  it  is  allowed  to  do  so,  that  is,  the 


THE  IDEAL  STEAM  ENGINE  57 

high-pressure  steam  is  allowed  to  push  it  further  out.  This 
can  be  pictured  by  imagining  the  steam  to  act  like  the 
compressed  spring  shown  in  the  figure  and  to  push  the 
piston  in  much  the  same  way  as  does  the  spring.  The  line 
b\c\  shows  the  decreasing  pressure  exerted  on  the  piston  by 
the  spring  as  the  latter  expands  so  as  to  get  longer  and 
longer.  Because  of  the  properties  of  a  spring  this  is  a 
straight  line.  The  line  be  shows  the  decreasing  pressure 
exerted  on  the  piston  by  the  steam  as  the  latter  expands 
so  as  to  occupy  greater  and  greater  volumes.  Because  of 
the  properties  of  steam  this  line  is  curved  instead  of  straight. 

Work  will  be  done  on  the  piston  by  the  expanding 
steam  during  the  process  be  and  the  amount  of  this  work 
will  be  indicated  by  the  area  under  the  line  be  as  shown 
in  the  figure.  This  work  must  have  been  done  by  the 
expenditure  of  energy  on  the  part  of  the  steam  and  since 
no  energy  was  added  after  the  point  b  was  reached  the  work 
must  have  been  done  at  the  expense  of  heat  energy  contained 
in  the  steam  at  6.  It  has  already  been  shown  that  the 
heat  above  32°  in  the  steam  at  b  is  equal  to  the  sum  of  the 
heat  of  the  liquid  and  the  internal  latent  heat,  and  some 
of  this  heat  must  obviously  be  used  for  the  doing  of  work 
along  be  instead  of  being  entirely  rejected  to  the  cooling 
medium  as  in  the  preceding  cycle  without  "  expansion." 

The  expansion  of  the  steam  continues  until  the  "  back 
pressure  "  P^  is  reached.  The  cooling  medium  may  then 
be  imagined  to  be  brought  into  use  and  to  abstract  such 
heat  of  vaporization  as  may  remain  in  the  steam  besides 
absorbing  the  equivalent  of  the  work  done  on  the  steam 
by  the  returning  piston,  thus  giving  the  process  shown  by 
the  line  cd. 

If  the  expansion  line  be  of  the  cycle  just  described 
could  be  carried  out  within  walls  constructed  of  such  mate- 
rial that  it  would  not  give  heat  to  nor  take  heat  from 
the  steam,  it  is  obvious  that  any  heat  energy  lost  by  the 
steam  during  the  expansion  could  be  lost  only  by  conver- 


58  STEAM  POWER 

sion  into  mechanical  energy.  An  expansion  of  this  kind 
is  called  an  adiabatic  expansion. 

In  the  figure,  the  curve  of  adiabatic  expansion  is  shown 
in  its  correct  position  with  respect  to  the  saturation  curve 
and  it  is  obvious  that  for  an  adiabatic  expansion,  starting 
with  dry,  saturated  steam,  the  quality  decreases  as  the  expan- 
sion progresses. 

Comparison  with  Cycle  without  Expansion.  The  heat 
supplied  is  the  same  in  both  of  the  cycles  just  considered 
when  they  operate  between  the  same  two  pressures,  but 
the  mechanical  energy  obtained  in  the  case  of  the  complete 
expansion  cycle  is  much  greater.  In  Fig.  21,  for  instance, 
the  mechanical  energy  obtainable  with  the  cycle  first 
described  is  represented  by  the  area  abd'd  while  that  obtain- 
able with  the  complete  expansion  cycle  with  the  same 
heat  supply  Q\  is  represented  by  the  same  area  abd'd  plus 
the  additional  area  bed'.  The  efficiency  of  the  complete 
expansion  cycle  is  therefore  very  much  higher  than  that 
of  the  cycle  without  expansion. 

For  conditions  similar  to  those  giving  a  theoretical 
efficiency  of  about  6  per  cent  without  expansion,  the  com- 
plete expansion  cycle  will  give  a  theoretical  efficiency  of 
about  12  per  cent  and  this  figure  can  be  doubled  by 
expedients  which  will  be  considered  later. 

The  cylinder  required  for  the  production  of  the  com- 
plete expansion  cycle  would  be  much  larger  than  that  re- 
quired for  the  other  cycle  if  both  used  the  same  weight  of 
steam  per  cycle.  The  proportion  would  be  in  the  ratio 
of  the  volume  shown  at  c  in  Fig.  21  to  the  volume  shown 
at  6.  But  the  complete  expansion  cycle  would  make  avail- 
able much  more  energy  per  pound  of  steam  than  would 
the  other,  so  that  the  difference  in  the  size  of  cylinders 
would  not  be  so  great  if  both  were  required  to  make  avail- 
able the  same  amount  of  mechanical  energy  per  cycle. 

40.  The  Incomplete-expansion  Cycle.  The  shape  of 
this  cycle  is  shown  in  Fig.  22.  It  is  just  like  the  complete 


THE  IDEAL  STEAM  ENGINE 


59 


FIG.  22. — Incomplete  Expansion  Cycle. 


expansion  cycle  down  to  the  point  c.     The  cylinder  in  which 

it  is  produced  has  a  smaller  volume  than  that  used  for  the 

complete  expansion  cycle  so  that  the  piston  arrives  at  the 

end  of  its  stroke  before  it 

has    opened    up    volume 

enough     to     enable     the 

steam  to  expand   all  the 

way  down  to  the  lowest 

pressure  (terminal  or  back 

pressure) .  When  the  point 

c  is   reached   in  the   real 

engine,  the  exhaust  valve 

is    opened     and     enough 

steam  then  blows  out  to 

reduce  the  pressure  to  the  back  pressure  Pa.     The  piston 

then  returns  and  drives  out  the  remainder  of  the  steam  as 

shown  by  the  line  de. 

In  the  ideal  method  assumed  in  the  preceding  treat- 
ment, the  heat  absorbing  medium  would  be  brought  into 
use  at  c,  absorbing  sufficient  heat  to  reduce  the  pressure 
from  PC  to  Pd  while  the  piston  remained  stationary  at  the 
end  of  its  stroke.  The  latent  heat  of  vaporization  remain- 
ing in  the  steam  at  d  would  then  be  absorbed  as  the  piston 
was  driven  back  from  d  to  e. 

Comparison  with  Other  Cycles.  The  incomplete  expan- 
sion cycle  is  intermediate  between  the  two  previously  dis- 
cussed. This  can  be  appreciated  readily  by  an  inspection 
of  Fig.  22.  In  this  figure  the  area  abd'e  represents  the 
mechanical  energy  obtainable  with  the  cycle  without 
expansion;  the  area  abc'e  represents  the  energy  obtainable 
from  the  same  quantity  of  steam  with  complete  expansion; 
and  the  area  abcde  represents  the  energy  obtainable  from 
the  same  amount  of  steam  with  incomplete  expansion. 

The  later  the  point  at  which  the  exhaust  valve  is  opened, 
point  c,  the  more  nearly  do  efficiency  and  energy  obtain- 
able,approach  the  values  for  the  complete  expansion  cycle  „ 


60  STEAM  POWER 

The  earlier  the  point  at  which  the  exhaust  valve  is  opened, 
the  more  nearly  do  efficiency  and  energy  obtainable  approach 
the  values  for  no  expansion. 

Despite  the  lower  efficiency  of  the  incomplete  expan- 
sion cycle  as  brought  out  in  connection  with  Fig.  22  it  is 
universally  used  on  all  reciprocating  engines  excepting 
those  which  make  no  pretense  to  economy  and  use  no 
expansion.  The  less  efficient  cycle  is  used  for  the  simple 
reason  that  complete  expansion  in  a  reciprocating  engine 
does  not  pay  commercially.  For  complete  expansion  the 
cylinder  must  be  larger  in  the  ratio  of  Vc  to  Vc>  as  shown 
in  Fig.  22  and  the  work  obtained  by  completing  the  expan- 
sion is  a  very  small  part  of  the  total.  In  most  cases  it 
would  not  be  great  enough  to  overcome  the  friction  of  the 
engine,  not  to  mention  paying  interest  on  the  necessarily 
higher  cost  of  the  larger  cylinder  and  accompanying  parts. 

It  will  be  shown  in  a  later  chapter  that  the  steam  tur- 
bine can  economically  expand  the  steam  completely  and 
the  complete  expansion  cycle  is  therefore  used  with  such 
prime  movers. 


CHAPTER  V 
ENTROPY    DIAGRAM 

41.  Definitions.  In  Chapter  III  temperature,  pressure 
and  volume  were  discussed  as  criteria  determining  the  con- 
dition of  water  and  steam.  Other  things  may  be  used  in 
determining  the  condition  of  such  materials.  One  which  is 
particularly  useful  from  an  engineering  standpoint  is  known 
as  entropy  and  is  designated  by  the  Greek  letter  (/>. 

For  every  condition  of  water  and  steam,  there  is  a  char- 
acteristic value  of  entropy  just  as  there  is  a  characteristic 
value  of  temperature,  pressure,  volume,  heat  above  32°  F., 
etc.  These  values  of  entropy  are  given  in  the  steam  tables 
in  just  the  same  way  as  the  value  of  temperature,  pressure, 
volume,  heat  above  32°  F.,  and  such,  are  given. 

The  entropy  of  the  liquid  given  for  any  particular  pres- 
sure is  the  change  of  entropy  experienced  by  one  pound 
of  the  liquid  when  its  temperature  is  raised  from  32°  F. 
to  the  temperature  of  vaporization  corresponding  to  that 
particular  pressure.  It  might  be  spoken  of  as  the  entropy 
of  the  liquid  above  32°  F.,  just  as  q  is  spoken  of  as  the  heat 
of  the  liquid  above  32°  F.  It  is  represented  by  <fr. 

The  entropy  of  vaporization  given  for  any  particular 
pressure  is  the  change  of  entropy  experienced  by  one  pound 
of  the  material  while  changing  from  water  at  the  tempera- 
ture of  vaporization  to  dry  saturated  steam  at  constant 
pressure.  It  corresponds  to  the  latent  heat  of  vaporiza- 
tion and  is  designated  by  </>„. 

The  entropy  of  dry  saturated  steam  at  any  pressure  is 
the  sum  of  fa  and  4>v  and  therefore  is  the  total  change  of 
entropy  experienced  by  a  pound  of  material  in  changing 

61 


62 


STEAM  POWER 


from  water  at  32°  F.  to  dry  saturated  steam  at  the  particu- 
lar pressure  in  question. 

The  entropy  of  superheat  at  any  pressure  and  tempera- 
ture is  the  change  of  entropy  experienced  by  a  pound  of 
dry,  saturated  steam  at  that  pressure  when  superheated 
to  that  particular  temperature.  It  is  designated  by  </>s. 

The  entropy  of  superheated  stsam  at  any  pressure  and 
temperature  is  the  total  change  of  entropy  experienced  by 
one  pound  of  material  when  changed  from  water  at  32°  F. 


FIG.  23. — Temperature-Entropy  Diagrams. 

to  superheated  steam  at  the  pressure  and  temperature 
in  question.  It  is  equal  to  <&+<£„+</>«. 

42.  Temperature-Entropy  Chart  for  Steam.  Entropy 
is  particularly  useful  to  the  engineer  because  it  enables  him 
to  draw  charts  which  lend  themselves  readily  to  an  easy, 
graphical  solution  of  certain  problems  which  would  other- 
wise involve  complex  calculations. .  One  of  these  charts 
is  known  as  the  Temperature-Entropy  Chart. 

In  making  this  chart,  absolute  temperature  is  generally 
plotted  on  the  vertical  and  entropy  above  some  datum  tem- 
perature on  the  horizontal,  as  shown  in  Fig.  23  (a)  and  (6), 
which  represents  the  construction  of  a  temperature  entropy 
diagram  for  water  and  steam.  The  entropy  values  on 


ENTROPY  DIAGRAM  63 

this  chart  are   plotted  above  32°   F.   as   datum  tempera- 
ture. 

The  water  line  or  water  curve  is  obtained  by  picking 
out  of  the  steam  tables  the  values  of  fa,  entropy  of  the  liquid, 
for  different  pressures  and  plotting  them  against  the  abso- 
lute temperatures  corresponding  to  those  pressures.  Ob- 
viously, zero  of  entropy  will  occur  at  the  absolute  tempera- 
ture corresponding  to  32°  F.,  i.e.,  about  492°  F.  abs. 

The  saturation  curve  or  dry  steam  curve  is  obtained  by 
picking  out  of  the  steam  tables  the  values  of  fa-}- <j>v  for 
different  pressures  and  plotting  against  corresponding 
absolute  temperatures. 

The  entropy  of  vaporization  is  obviously  shown  for  each 
different  temperature  (or  pressure)  by  the  distance  between 
the  water  curve  and  the  saturation  curve,  since  the  former 
is  distant  from  the  vertical  axis  by  an  amount  equal  to  fa, 
while  the  latter  is  distant  an  amount  equal  to  fa-}- fa. 

Superheating  lines  are  drawn  by  picking  from  the  steam 
tables  the  values  of  entropy  above  32°  F.  for  steam  super- 
heated to  different  temperatures  at  one  particular  pressure 
and  plotting  against  the  proper  temperatures.  There  will 
be  as  many  superheating  lines  on  the  diagram  as  one  chooses 
pressures  for  which  to  plot  them.  Only  one  is  shown 
in  the  figure. 

One  very  useful  property  of  this  diagram  follows  from  the 
fact  that  points  on  its  surface  indicate  the  condition  of  the 
material.  For  instance,  if  the  temperature-entropy,  or 
T  —  <£,  values  of  the  material  at  a  given  condition  should 
plot  to  the  left  of  the  liquid  line,  the  material  must  be  in  the 
liquid  condition;  if  they  plot  between  the  liquid  line  and  the 
saturation  curve,  the  material  must  be  a  mixture  of  liquid 
and  saturated  vapor;  if  they  plot  on  the  saturation  curve, 
the  material  must  be  dry,  saturated  steam;  and  if  they 
plot  to  the  right  of  the  saturation  curve,  the  material  must 
be  superheated  steam.  This  all  follows  directly  from  the 
definition  of  entropy  above  32°  F.,  as  plotted  in  these  dia- 


64  STEAM  POWER 

grams.  The  various  regions,  or  fields,  into  which  the  dia- 
gram divides  in  this  way  are  shown  in  Fig.  23  (a). 

Another  very  useful  property  of  this  diagram  follows 
from  the  fact  that  area  represents  heat  just  as  area  on  a 
pressure-volume  diagram  was  found  to  represent  work. 
Thus  the  area  under  the  line  ab,  for  instance,  represents 
the  heat  required  to  raise  the  temperature  of  one  pound 
of  water  from  32°  F.  to  the  temperature  at  b.  Similarly 
the  area  under  the  line  be  represents  the  heat  required  to 
change  a  pound  of  water  at  the  temperature  at  6  to  a  pound 
of  dry,  saturated  steam  at  the  same  temperature.  The 
heat  required  to  superheat  this  pound  of  saturated  steam 
at  constant  pressure  up  to  the  temperature  shown  at  d 
is  similarly  represented  by  the  area  under  the  line  cd. 

In  this  connection,  it  should  be  noted  that  this  diagram 
is  plotted  above  absolute  zero  of  temperature  just  as  the 
pressure-volume  diagram  is  plotted  above  absolute  zero  of 
pressure.  The  areas  in  question  therefore  extend  down 
to  the  absolute  zero  of  temperature.  In  order  to  indicate 
this  in  Fig.  23  (b),  a  large  part  of  the  chart  is  supposed  to 
have  been  broken  out,  so  that  the  lower  end  of  the  diagram 
could  be  moved  up  into  view.  In  Fig.  23  (a),  the  bottom 
of  the  diagram  is  drawn  a  few  degrees  below  32°  F.  and 
this  is  indicated  by  putting  T>0  opposite  the  horizontal 
axis. 

The  various  areas  hatched  in  Fig.  23  (6)  indicate  the 
various  quantities  of  heat  previously  discussed.  It  should 
be  understood  that  the  areas  represent  the  heat  quantities 
only  for  the  particular  pressure  which  corresponds  to  the 
temperature  indicated  by  Tv.  For  a  higher  pressure,  the 
line  be  would  be  higher  and  the  areas  proportionately 
larger;  for  a  lower  pressure  the  line  be  would  be  lower  and 
the  areas  smaller. 


ENTROPY  DIAGRAM 


65 


ILLUSTRATIVE  PROBLEM 

Starting  with  liquid  at  a  temperature  Tt  corresponding  to  the 
temperature  of  vaporization  at  a  pressure  of  50  Ibs.  per  square 
inch  absolute,  assume  the  liquid  raised  to  the  temperature  of 
vaporization  at  a  pressure  of  100  Ibs.  per  square  inch  absolute 
and  then  completely  vaporized.  Determine  the  various  changes 
of  entropy  and  indicate  them  on  a  TV-chart. 

The  steam  tables  give  entropy  of  the  liquid,  4i,  as  equal  to 
0.4113  for  water  about  to  vaporize  under  50  Ibs.  per  sq.  in. 
absolute,  and  0.4743  for  water  about  to  vaporize  under  a  pres- 
sure of  100  Ibs.  per  sq.  in.  absolute.  The  difference,  that  is, 
0.4743-0.4113=0.0630,  must  be 
the  entropy  change  experienced 
by  the  liquid  when  its  tempera- 
ture is  raised  from  the  lower  to 
the  higher  value.  These  values 
itre  shown  in  Fig.  24. 

The  steam  tables  give  entropy 
of  vaporization,  <£»,  at  100  Ibs.  per 
square  inch  absolute  as  1.1277. 
Adding  this  to  the  entropy  above 
32°  F.  of  the  liquid  at  vaporiza- 
tion temperature  under  100  Ibs. 
pressure  gives  0.4743+1.1277  = 
1.602  as  the  entropy  above  32° 
of  dry,  saturated  steam  at  100 

Ibs.  per  square  inch  absolute.    These  values  are  all  indicated  in 
their  proper  position  in  Fig.  24. 

The  total  change  of  entropy  experienced  by  the  material  in 
changing  from  water  at  the  temperature  of  vaporization  under 
50  Ibs.  pressure  to  dry,  saturated  steam  at  100  Ibs.  pressure  is 
obviously  equal  to  0.0630+ 1.1277  =1.1907. 

43.  Quality  from  T^-chart.  The  entropy  change  ex- 
perienced by  steam  in  the  process  of  vaporization  is  directly 
proportional  to  the  addition  of  heat.  Thus,  when  half 
the  latent  heat  has  been  added  to  one  pound  of  material, 
the  entropy  change  is  J0».  In  general,  if  a  fraction  x  of 
the  latent  heat  has  been  added,  the  entropy  change  has 
been  x<t>f  during  the  process.  Therefore,  if  the  temperature 
entropy  condition  of  a  pound  of  material  should  plot  at  a 


FIG.  24. 


66 


STEAM  POWER 


point  such  as  c  in  Fig.  25,  it  follows  that  the  material  is 
a  mixture  of  water  and  steam  and  that  a  fraction  of  the 

be 

pound  equal  to  —  is  steam,  the  rest  being  water.     But, 
oct 

be 
by    definition,    the    fraction   —   is   x,    the  quality  of   the 

material. 

The  temperature-entropy  chart  is  very  useful  when  used 
in  connection  with  this  property  of  showing  quality.  Thus, 
in  Fig.  25,  the  area  under  be,  down  to  absolute  zero  tem- 
perature, represents  the  fraction  of  the  latent  heat  of 


Entropy 


FIG.  25. — Quality  irom  Temperature-         FIG.  26. — Constant  Quality 
Entropy  Chart.  Curves. 


vaporization  per  pound  which  must  be  added  to    give   a 
pound  the  quality  x. 

For  convenience  in  use,  constant  quality  lines  are 
generally  drawn  on  temperature-entropy  charts.  Such 
lines  are  shown  in  Fig.  26.  Each  line  is  obtained  by  plot- 
ting the  temperature  entropy  conditions  for  a  given  quality 
at  different  pressures.  For  this  purpose,  <f>v  and  <fr  are 
taken  from  the  steam  tables  for  a  given  pressure.  The 
numerical  value  of  <j>v  is  then  multiplied  by  the  fraction  re- 
presenting the  chosen  quality,  say  0.9,  and  the  product 
is  added  to  <£/,  giving  the  total  entropy  above  32°  F.  for 
quality  0.9  at  the  particular  pressure  chosen.  The  same 


ENTROPY  DIAGRAM 


67 


(M 


68  STEAM  POWER 

process  is  repeated  with  the  same  value  of  the  quality, 
but  with  different  pressures,  until  enough  points  have  been 
secured  to  make  it  possible  to  draw  a  smooth  line  through 
them. 

44.  Volume  from  T0-chart.      Since  quality  changes  at 
any  given  temperature,   or  pressure,   are  accompanied  by 
volume  changes,  it  is  possible  to  find  a  series  of  values  for 
the  quality  of  a  pound  of  wet  steam  which  will  make  that 
pound  occupy  the  same  volume  at  different  temperatures. 
Having  found  the  quality  which  will  be  necessary  at  a  num- 
ber of  different  temperatures,  the  total  entropy  above  32° 
F.  can  be  found  for  each  case  and  these  values  can  then  be 
plotted  on  the  T^-chart.     Connecting  the  points  so  obtained 
would  give  what  is  known  as  a  Constant  Volume  Line. 

Several  of  these  constant  volume  lines  are  shown  in  their 
correct  positions  in  Fig.  27.  It  will  be  observed  that, 
for  each  volume,  the  quality  must  increase  as  temperature 
(and  pressure)  increases  in  order  to  maintain  a  constant 
value  for  the  volume  occupied  by  one  pound  of  mixture. 

45.  Heat  from  T0-chart.      Equations  for  obtaining  the 
total  heat  above  32°  F.  for  wet  and  for  superheated  steam 
were    given    in    an    earlier    chapter.     By   means    of   these 
equations,  it  is  possible  to  find  a  succession  of  values  for 
quality  and  superheat  which  will  give  a  pound  of  material 
any   chosen   heat   content   at   different   pressures.     If   the 
corresponding  values  of  temperature  and  entropy  are  found 
and  plotted,  what  is  known  as  a  Constant  Heat  Line  results. 
Several  of  these  lines  are  shown  in  Fig.  27. 

46.  The  Complete  T(£-chart  for  Steam.     A  very  com- 
plete, graphical  representation  of  the  properties  of  water 
and  steam  can  be  procured  by  combining  in  one  diagram 
all  of  the  lines  discussed  in  preceding  paragraphs.     Such 
a  diagram  is  generally  spoken  of  as  the  T ^-diagram  or  the 
T<f>-chart  for  steam.      An   example   of  such   a   diagram   is 
given  in  Fig.  28. 

This  chart  is  very  useful,  as  it  enables  one  to  solve  by 


•ui  -bg  'sqi)  oanssaaj  a^nTOsqy 


70  STEAM  POWER 

inspection  many  of  the  most  difficult  problems  which  arise 
in  the  theory  and  practice  of  using  steam.  As  an  example, 
assume  that  it  is  desirable  to  know  what  will  happen  if 
water  at  the  temperature  of  vaporization  corresponding 
to  about  24  Ibs.  per  square  inch  absolute  has  its  volume 
increased  indefinitely  at  constant  temperature.  The  initial 
condition  of  the  water  would  be  shown  on  the  water  curve 
of  Fig.  28  at  the  point  at  which  the  700°  absolute  temperature 
line  crosses  it.  Increase  of  volume  at  constant  temperature 
would  be  indicated  by  a  horizontal  line  running  to  the  right 
from  this  point.  Obviously,  vaporization  will  occur  at 
constant  pressure  (because  the  temperature  is  constant) 
and  the  quality  will  change  from  zero  to  unity  at  which  the 
saturation  curve  will  have  been  reached.  Further  increase 
of  volume  can  result  only  in  the  production  of  superheated 
steam,  since  the  line  representing  the  process  will  rur\ 
out  into  the  superheated  steam  field.  It  is  also  interesting 
to  note  that  the  pressure  on  the  material  will  have  to  be 
decreased  as  the  volume  increases  in  the  superheated  steam 
region,  as  is  evidenced  by  the  fact  that  the  horizontal  line 
representing  the  assumed  process  cuts  lower  and  lower 
pressure  lines  as  it  is  extended  to  the  right  in  the  super- 
heated field. 

Note  also  that  the  intersections  of  this  horizontal  line 
with  constant  volume  and  constant  heat  lines  afford  the 
means  of  determining  volume  and  heat  above  32°  F.  at 
different  stages  of  the  assumed  process. 

PROBLEMS 

1.  Determine  from  the  steam  tables  the  change  of  entropy 
experienced  by  one  pound  of  water  when  its  temperature  is  raised 
from  32°  F.  to  the  temperature  of  vaporization  under  a  pressure 
of  100  Ibs.  per  square  inch  absolute. 

2.  Determine  from  the  steam  tables  the  entropy  change  experi- 
enced by  one  pound  of  water  when  its  temperature  is  raised  from 
32°  F.  to  the  temperature  of  vaporization  under  a  pressure  of 
150  Ibs.  per  square  inch  absolute. 


ENTROPY  DIAGRAM  71 

3.  Determine  the  entropy  change  experienced  by  one  pound  of 
water  when  its  temperature  is  raised  from  the  temperature  of 
vaporization  corresponding  to  100  Ibs.  per  square  inch  to  that 
corresponding  to  150  Ibs.  per  square  inch  by  subtracting  the  value 
found  in  Prob.  1  from  that  found  in  Prob.  2. 

4.  Determine  the  change  of  entropy  experienced  by  one  pound 
of  material  completely  vaporizing  at  a  temperature  of  327.8°  F. 

5.  Plot  a  TV-chart  for  one  pound  of  water.     Start  by  plotting 
entropy  of  the  liquid  for  various  temperatures;   then  plot  entropy 
of  saturated  steam  (above  32°  F.);  finally  draw  water  line,  satura- 
tion line,  and  several  lines  showing  change  of  entropy  during  vapor- 
ization. 

6.  Determine  from  a   TV-chart  the  quality  which  would  be 
attained  by  one  pound  of  steam  if  it  experienced  a  change  which 
carried  it  from  the  condition  of  dry  saturated  steam  at  150  Ibs. 
per  square  inch  absolute  to  a  pressure  of  25  Ibs.  per  square  inch 
absolute  by  a  process  which  would  plot  as  a  vertical  line  on  the 

TV-chart. 

7.  Assume  a  pound  of  mixed  water  and  steam  to  have  a  qual- 
ity of  80%  at  a  pressure  of  200  Ibs.  per  square  inch  absolute. 
Determine  from  the  TV-chart  the  heat  above  32°  per  pound  of 
mixture  and  the  volume  occupied  by  the  mixture.     Determine 
also  the  quality  attained  if  the  pressure  of  the  material  drops  to 
20  Ibs.  per  square  inch  absolute  at  constant  entropy.     How  does 
the  heat  above  32°  F.  change  during  such  a  process? 

8.  Assume  a  pound   of    mixture    as   in    Prob.  7,  but  with  a 
quality  of  30%  at  a  pressure  of  200  Ibs.     Find  all  quantities  called 
for  in  that  problem. 

9.  Assume  a  pound   of   material  as  in  Probs.  7  and  8  above, 
but  superheated  200°  at  a  pressure  of  200  Ibs.  per  square  inch 
absolute.     Determine  all  quantities  called  for  in  Prob.  7. 

10.  Choose  a  point  on  the  TV-chart  at  which  a  constant  volume 
line  intersects  the  saturation  curve.     Determine  the  change  of 
quality,  entropy  and  heat  above  32°  F.,  if  the  material  drops 
to  half  pressure  at  constant  volume. 


CHAPTER  VI 


TEMPERATURE   ENTROPY  DIAGRAMS   OF   STEAM 
CYCLES 

47.  Complete  Expansion  Cycle.  This  cycle  was  con- 
sidered in  Chapter  IV  and  the  PF-diagrarn  was  given  there 
as  Fig.  21.  The  diagram  of  this  cycle  drawn  to  T^-co- 
ordinates  is  shown  in  Fig.  29.  The  same  letters  are  used 

to  represent  corresponding  points 
in  the  two  diagrams. 

The  entropy  change  during 
the  heating  of  the  liquid  is 
shown  by  the  part  of  the  liquid 
line  between  d  and  a,  and  the 
heat  supplied  during  that  process 
is  represented  by  the  area  below 
the  line  da,  measuring  clear 
down  to  the  absolute  zero  of 
temperature. 

The   entropy  change    during 


FIG.  -9. — 7>-diagram,  Com- 
plete Expansion  Cycle. 


vaporization  is  represented  by  the  line  ab  and  the  heat 
supplied  during  the  process  is  shown  by  the  total  area 
under  that  line. 

The  adiabatic  expansion  of  the  steam  is  represented  by 
the  line  be,  such  an  adiabatic  change  fortunately  being  a  con- 
stant entropy  process  and  therefore  easily  drawn  in  this 
diagram.  Obviously  no  heat  is  received  or  removed  dur- 
ing this  process,  as  there  is  no  area  under  the  line  be. 

The  entropy  change  during  condensation  is  represented 
by  the  line  cd  and  the  heat  rejected  by  the  working  sub- 
stance during  this  process  is  represented  by  the  area  under 
that  line. 

72 


TEMPERATURE  ENTROPY  DIAGRAMS  73 

48.  Area  of   Cycle   Representative   of  Work.     It   will 
be  remembered  that  area  under  a  line  in  the  PF-diagram 
represents  work  in  foot-pounds.     That  diagram,  however, 
gi^es  no  indication  of  heat  received  or  rejected  and  it  is 
not  possible  to  obtain  any  direct  idea  of  efficiency  from  it. 
In  this  respect,  the  TV-diagram  is  much  better.     Area  under 
the  lines  da  and  ab  in  Fig.   29  represents   heat  supplied 
the  working  substance.     Area  under  the  line  cd  represents 
heat  rejected   by  the  working  substance.     The  difference 
between  these  two,  or  the  area  enclosed  within  the  lines 
of  the  cycle,  must  therefore  represent  the  heat  converted 
into  mechanical  energy  per  cycle. 

This  diagram  therefore  shows  directly  by  areas  the 
heat  supplied,  the  heat  rejected,  and  the  heat  converted 
into  mechanical  energy.  Further,  the  ratio  of  the  area 
representing  heat  converted  into  work,  and  the  area  repre- 
senting heat  supplied  must  be  the  efficiency  of  the  cycle. 

Remembering  also  that  if  the  lines  of  the  cycle  are  drawn 
upon  a  T^-chart  such  as  that  given  in  Fig.  28,  all  volume 
changes,  heat  contents  and  qualities  at  different  points 
are  shown  without  further  work,  it  becomes  evident  that 
this  form  of  representation  is  decidedly  convenient  and  far 
superior  to  the  pressure  volume  method. 

49.  Modifications   for   Wet    and    Superheated    Steam. 
The  complete  expansion  cycle  is  supposed  to  represent  an 
idealization  of  what  happens  in  a  real  prime  mover.     In  real 
cases,  however,  the  steam  may  arrive  at  the  prime  mover  wet 
or  superheated  and  it  is  desirable  to  investigate  the  method 
of  representing  such  conditions  as  well  as  their  effects. 

Wet  steam  corresponds  to  incomplete  vaporization, 
i.e.,  a  quality  less  than  unity  at  the  upper  right-hand  corner 
of  the  cycle.  This  might  be  shown  for  a  given  case  by  the 
location  of  the  point  6'  in  Fig.  29.  The  cycle  would  then 
be  ab'c'd  and  a  smaller  amount  of  work  would  be  obtained 
per  pound  of  working  substance  as  evidenced  by  the  smaller 
area  enclosed  within  the  lines  of  the  cycle. 


74 


STEAM  POWER 


In  the  case  of  superheated  steam,  superheating  occurs 
at  constant  pressure  .after  vaporization  is  complete.  This 
would  be  shown  by  the  location  of  the  upper  right-hand 
corner  of  the  cycle  at  some  point  6"  on  the  constant  pressure 
line  which  extends  out  from  b.  The  cycle  is  now  represented 
by  abb"c"d  and  evidently  has  a  different  shape  than  it 
had  in  the  preceding  cases.  Obviously  the  area  enclosed 
within  the  lines  of  the  cycle  is  greater  than  it  was  before  and 
therefore  more  mechanical  energy  is  obtained  per  pound 
of  steam. 

50.  Incomplete  Expansion  Cycle.  The  only  difference 
between  the  incomplete  and  complete  expansion  cycles  is 


FIG.  30. — TV-diagram,  Incom- 
plete Expansion  Cycle. 


FIG.  31. — 7'0-diagram,  Cycle 
Without  Adiabatic  Expansion. 


the  termination  of  the  expansion  in  the  former  by  means  of 
a  constant  volume  line.  This  is  shown  to  TV-coordinates 
in  Fig.  30  in  which  the  incomplete  expansion  cycle  is  drawn 
in  heavy  lines  over  the  one  in  which  expansion  continues 
to  the  back  pressure. 

The  constant  volume  line  is  seen  to  cut  off  a  corner, 
thus  reducing  the  area  representing  heat  converted  into 
work.  The  heat  supplied  in  each  case  is  measured  by  the 
area  under  the  lines  ea  and  ab.  The  efficiency  of  the  cycle 
with  incomplete  expansion  can  therefore  be  seen  to  be  less 
than  that  of  the  other  cycle  by  simple  inspection  of  the 
diagram. 

If  the  adiabatic  expansion  is  terminated  at  a  higher 


TEMPERATURE  ENTROPY  DIAGRAMS  75 

pressure,  as  by  the  constant  volume  line  c"d"  in  Fig.  30, 
still  more  of  the  work  area  is  lost,  but  the  same  quantity 
of  heat  is  supplied,  and  therefore  the  efficiency  is  still  lower 
than  when  the  expansion  terminated  at  c.  Obviously 
as  the  point  at  which  the  adiabatic  expansion  is  terminated 
moves  nearer  and  nearer  to  b  as  shown  in  Pig.  31,  the  cycle 
becomes  less  and  less  efficient.  If  the  constant  volume 
line  starts  at  b,  there  is  no  adiabatic  expansion  and  the 
cycle  becomes  that  previously  considered  as  having  a  rec- 
tangular shape  in  the  PF-diagram.  This  cycle  has  the  shape 
indicated  by  abed  in  the  TV-diagram  of  Fig.  31.  Obviously 
it  is  least  efficient  of  all  as  was  previously  shown  by  other 
means. 

51.  Effect  of  Temperature  Range  on  Efficiency.  It  has 
already  been  stated  (see  p.  26)  that  heat  engines  receive 
heat  at  a  high  temperature,  convert  some  of  it  into  me- 
chanical form  and  discharge  the  remainder  at  a  lower  tem- 
perature. Inspection  of  the  TV-diagram  shows  this  very 
clearly,  and,  remembering  that  the  area  of  the  cycle  measures 
the  heat  converted,  these  diagrams  also  show  how  raising 
the  upper  temperature  (or  pressure)  or  lowering  the  lower 
temperature  (or  pressure)  will  increase  the  efficiency.  It 
can  be  seen  readily  that  lowering  the  lower  temperature 
will,  however,  be  more  effective  in  increasing  the  efficiency 
than  raising  the  upper  temperature. 

PROBLEMS 

4-  1.  Draw  a  complete  expansion  cycle  to  T ^-coordinates  for  the 

following  conditions  (using  TV-diagram  for  steam  to  get  values); 

weight  of  working    substance,    1    lb.;    initial    pressure,    125   Ibs. 

absolute;    quality     at  beginning  of  adiabatic   expansion,    100% 

back  pressure,  10  Ibs.  absolute. 

2.  Determine  the  following  values  for  cycle  drawn  in  Prob.  1 : 
(a)  Entropy  of  liquid  at  beginning  of  vaporization; 
(6)  Entropy  at  beginning  of  adiabatic  expansion; 

(c)  Quality  at  end  of  adiabatic  expansion; 

(d)  Volume  at  end  of  adiabatic  expansion- 

(e)  Entropy  at  end  of  condensation. 


76  STEAM  POWEP 

-T  3.  Show  by  measuring  the  area  on  TV-diagrams,  the  increase  of 
efficiency  resulting  from  the  use  of  an  initial  pressure  of  175  Ibs. 
absolute  and  from  the  use  of  a  terminal  pressure  of  2  Ibs.  absolute 
in  place  of  the  values  given  in  Prob.  1. 

•^  4.  Compare  the  efficiency  of  a  complete  expansion  cycle  with 
conditions  as  in  Prob.  1  with  a  complete  expansion  cycle  with 
same  pressures  but  with  a  temperature  of  500°  F.  at  the  beginning 
of  the  adiabatic  expansion. 

5.  Draw   an   incomplete   expansion   cycle    to    T ^-coordinates 
for  the  same  pressures  as  in  Prob.  1,  but  with  adiabatic  expan- 
sion ending  at  a  pressure  of  15  Ibs.  absolute. 

6.  Compare  work  and  efficiency  of  the  two  cycles  of  Probs. 
1  and  5  above. 

7.  Draw  a    cycle  without    expansion  for   the   conditions    of 
Prob.  1  to  TV-coordinates  and  compare  the  work  area  with  that 
obtained  in  Probs.  1  and  5. 


CHAPTER  VII 
THE   REAL  STEAM  ENGINE 

52.  Operation  of  Real  Engine.  In  previous  chapters 
the  ideal  steam  engine  was  considered  and  several  cycles 
upon  which  it  might  be  operated  were  discussed.  Real 
engines  are  built  to  operate  on  the  same  cycles,  but  because 
of  certain  practical  considerations  they  only  imperfectly 
approximate  the  ideal  performance. 

Real  engines  must  be  built  of  iron  and  steel  for  practical 
reasons  and  these  metals  absorb,  conduct  and  radiate  heat 
so  that  certain  heat  interchanges  between  the  working 
substance  and  engine  and  certain  heat  losses  occur  in 
practical  operation.  These  were  eliminated  in  the  ideal 
case  by  simply  assuming  ideal  materials  not  possessed 
of  the  characteristics  of  real  metals. 

It  is  also  practically  impossible  to  generate  steam 
in  the  cylinder  of  a  real  engine  as  was  assumed  to  be  done 
in  the  ideal  case.  Heat  is  practically  obtained  by  the  com- 
bustion of  fuels,  and  the  higher  the  temperature  attained 
the  better  can  the  liberated  heat  be  utilized  in  the  genera- 
tion of  steam.  To  subject  the  cylinder  to  such  high  tem- 
peratures and  to  control  the  heating  and  cooling  as  neces- 
sary to  produce  a  number  of  cycles  in  rapid  succession  would 
lead  to  rapid  wear  and  great  practical  difficulties.  It  has 
been  found  best  to  generate  the  steam  in  a  boiler  which  is 
properly  equipped  for  that  purpose  and  then  to  transmit 
it  with  its  contained  heat  to  the  engine,  which  is  constructed 
in  such  a  way  as  to  utilize  that  heat  to  the  best  advantage. 
If  the  steam  is  to  be  condensed,  as  assumed  in  the  ideal 
cases,  it  has  also  been  found  best  to  remove  it  from  the 

77 


78 


STEAM  POWER 


cylinder  and  to  condense  it  in  a  separate  piece  of  apparatus 
properly  constructed  for  that  purpose. 


The  entire  arrangement  which  results  from  these  prac- 
tical modifications  in  the  case  of  a  non-condensing  engine 


THE  REAL  STEAM  ENGINE  79 

is  shown  in  Fig.  32.  Steam  is  generated  within  the  boiler 
at  some  constant  pressure  PI  and  at  the  proper  instant  the 
admission  valve  at  one  end  of  the  cylinder  is  opened,  allow- 
ing steam  to  flow  in  and  drive  the  piston  outward.  If 
there  were  no  losses,  this  would  be  represented  by  some 
such  line  as  ab  at  a  height  PI  on  the 
PF-diagram  of  Fig.  33.  Closing  of  pi 
the  valve  after  the  piston  had  moved 
part  way  out  would  cut  off  the 
further  flow  of  steam,  and,  with  con- 
tinued motion  of  the  piston,  the  steam 
within  the  cylinder  would  expand.  FIQ  33 

If     no    heat     interchanges    occurred, 
this  expansion  be  would  be  adiabatic  as  in  the  real  case. 

It  will  be  observed  that  the  two  lines  on  the  PF-diagram 
thus  far  produced  represent  equally  well  the  corresponding 
two  lines  of  the  complete  or  incomplete-expansion  cycles. 
The  heat  supplied  in  the  boiler  is  the  same  as  that  supplied 
in  the  cylinder  under  the  ideal  conditions  originally  assumed, 
and  the  work  under  the  line  a b  is  equal  to  the  external 
work  done  during  vaporization  just  as  in  the  ideal  case. 
If  difficulty  is  experienced  in  connection  with  the  statement 
regarding  external  work,  it  is  only  necessary  to  picture  the 
process  in  this  way:  Assume  that  each  pound  of  steam 
formed  in  the  boiler  does  the  external  work  equivalent  to 
A Pu  by  pushing  the  pound  previously  generated  ahead  of 
it  as  a  piston,  and  that  this  motion  communicated  along 
the  pipe  from  layer  to  layer  results  in  pushing  an  equivalent 
weight  (and  volume)  into  the  cylinder  against  the  resist- 
ance offered  to  the  piston's  motion. 

When  the  piston  arrives  at  the  end  of  its  stroke  at  the 
point  c,  the  opening  of  the  "  exhaust  valve,"  connecting 
the  interior  of  the  cylinder  with  the  space  in  which  the 
pressure  P^  lower  than  Pc  is  maintained,  will  permit  some 
of  the  steam  to  blow  out  of  the  cylinder  with  the  piston 
standing  stationary  at  the  end  of  its  stroke.  This  would 


gO  STEAM  POWER 

give  a  constant  volume  change  equivalent  to  the  correspond- 
ing line  in  the  incomplete-expansion  cycle. 

The  return  of  the  piston  from  d  to  e,  with  the  exhaust 
valve  still  open,  would  force  the  remainder  of  the  steam 
out  of  the  cylinder  and  into  the  space  in  which  the  pres- 
sure P2  is  maintained.  The  result,  so  far  as  the  diagram 
is  concerned,  is  obviously  the  same  as  in  the  ideal  case,  and 
if  the  steam  were  condensed  within  a  proper  vessel  into 
which  it  exhausted  (instead  of  being  exhausted  to  atmos- 
phere), the  result  would  also  be  the  same  so  far  as  the  shape 
of  the  diagram  is  concerned.  The  pressure  P^  might,  how- 
ever, be  maintained  at  a  lower  value,  thus  giving  a  greater 
temperature  range. 

The  pressure  rise  ea  within  the  cylinder  would  result 
directly  from  the  opening  of  the  admission  valve  and  the 
admission  of  steam  for  the  next  cycle.  But,  if  the  working 
substance  is  to  be  returned  to  starting  conditions  as  was 
dene  in  the  ideal  case,  its  pressure  must  also  be  raised  to 
Pi  and  its  temperature  to  a  corresponding  value.  The 
pressure  is  raised  in  the  case  of  condensing  operation  by 
means  of  the  boiler  feed  pump,  which  picks  up  the  condensed 
steam  (condensate)  and  forces  it  into  the  boiler.  The 
temperature  of  the  working  substance  is  raised  by  passing 
it  through  feed-water  heaters  or  by  heat  absorbed  directly 
from  the  heated  water  in  the  boiler. 

When  operating  non-condensing  the  working  substance 
exhausted  during  the  last  part  of  each  cycle  is  really  thrown 
away  by  allowing  it  to  mix  with  the  atmosphere,  but  an 
equivalent  quantity  of  water  is  fed  to  the  boiler  by  the 
boiler  feed  pump  and  takes  the  place  of  the  material  lost 
by  exhaust  to  atmosphere.  This  method  of  operating 
does  not  approximate  the  ideal  as  closely  as  does  the  con- 
densing method,  but  the  discrepancy  is  not  very  great. 

53.  Losses  in  Real  Installations.  The  diagram  given 
in  Fig.  33  was  obtained  by  assuming  the  absence  of  certain 
practical  losses  and  is  considerably  modified  when  real 


Adlatattc  Ezpanilon 
of  Mixture  at  Point 
of  Cut-off. 

Adiabatic  Expansion  of 
all  Material  in  Cylinder 
if  Initially  Dry  and 


Back  Pressure 


^Saturated. 
C 


VoL 


THE  REAL  STEAM  ENGINE  81 

apparatus  is  used.  Thus  the  real  engine,  as  shown  in  con- 
nection with  Fig.  9,  has  clearance  and  operates  with  com- 
pression so  that  the  clearance  is  filled  with  steam  at  a  pres- 
sure indicated  by  the  point  a'  in  Fig.  34  when  the  admission 
valve  opens. 

There  is  also  always  some  drop  of  pressure  along  the 
steam  pipe  so  that  the   pressure 
at  the  engine  is  lower  than  at  the 
boiler.     Further,   the   admission 
valve  can  never  be  made  to  give 
such    a  large    opening    into  the 
cylinder    that    there    is    not    a 
measurable   drop  of  pressure  in   FlG.  34.-Theoretical  and  Real 
flowing  through  it.      As   a  result  Indicator  Diagrams, 

of    these     actions    the    highest 

pressure  attained  within  the  cylinder  as  indicated  at  point 
a  in  Fig.  34  is  always  lower  than  the  boiler  pressure  PI. 

As  the  piston  of  a  real  engine  moves  out  it  acquires 
a  higher  and  higher  velocity  until  it  reaches  a  point  near 
mid-stroke.  The  entering  steam  therefore  must  flow 
through  the  valve  with  increasing  velocity  if  it  is  to  follow 
up  the  piston  and  fill  the  cylinder,  but  this  usually  neces- 
sitates greater  pressure  drops  as  the  piston  moves  out, 
so  that  the  admission  line  generally  slopes  downward  instead 
of  being  horizontal.  There  is  also  another  phenomenon 
which  causes  this  line  to  slope.  The  metal  of  cylinder, 
cylinder  head  and  piston  is  in  contact  with  comparatively 
low-temperature  steam  during  the  latter  part  of  each  cycle, 
and  therefore  acquires  a  lower  temperature  than  that 
of  the  steam  about  to  enter.  Therefore,  when  the  high- 
pressure  steam  enters  the  cylinder  it  gives  up  heat  to 
the  walls  at  a  comparatively  rapid  rate,  and,  if  initially 
dry  saturated,  this  results  in  a  great  deal  of  condensation. 
Such  condensation  is  called  initial  condensation. 

As  the  steam  condenses  after  flowing  into  the  cylinder 
and  forms  water  occupying  a  negligibly  small  volume, 


82  STEAM  POWER 

it  follows  that  steam  must  flow  into  the  cylinder  at  a  pro- 
portionately greater  rate  in  order  to  fill  the  space  vacated 
by  the  piston.  But  this  results  in  an  increased  pressure 
drop  and  therefore  would  give  a  sloping  admission  line. 

When  the  piston  has  finally  been  driven  out  as  far  as 
desirable  by  the  action  of  high-pressure  steam,  the  admis- 
sion valve  is  closed,  that  is,  cut-off  occurs.  This  valve  can 
not  be  closed  suddenly;  the  closure  is  more  or  less  gradual 
in  all  cases.  As  the  opening  becomes  smaller  it  becomes 
increasingly  more  difficult  for  the  steam  to  flow  through 
and  into  the  cylinder  so  that  the  pressure  continues  to  drop 
at  an  increasing  rate  until  the  valve  is  finally  closed.  This 
gives  the  rounded  cut-off  shown  at  the  point  b. 

The  loss  of  pressure  during  admission  is  generally 
said  to  be  due  to  throttling  or  wire  drawing,  these  terms 
being  intended  to  convey  the  idea  that  the  steam  has  to 
squeeze  its  way  through  the  inlet  openings  with  correspond- 
ing loss  of  pressure. 

When  the  cut-off  has  finally  been  completed,  it  leaves 
the  end  of  the  cylinder  filled  with  a  mixture  of  steam  and 
water  at  steam  temperature,  and  this  mixture  then  expands 
as  shown  by  the  line  be.  At  the  beginning  of  the  expansion 
the  steam  generally  has  a  higher  temperature  than  that  of 
the  surrounding  walls  and  it  therefore  continues  to  give 
heat  to  those  walls.  Were  the  expansion  adiabatic  it 
would  follow  the  clot-dash  line  in  the  figure,  but,  as  the 
steam  must  not  only  convert  heat  into  work,  but  must  also 
supply  heat  to  the  walls,  it  condenses  more  rapidly  than 
in  the  ideal  case  and  its  pressure  and  volume  changes  follow 
some  such  law  as  that  indicated  by  the  upper  part  of  the 
curve  be. 

As  expansion  continues,  the  pressure  and  temperature 
of  the  steam  drop  until  some  point  is  reached  at  which 
the  temperature  has  become  equal  to  that  of  the  walls. 
Further  expansion  with  drop  of  pressure  and  temperature 
results  in  reducing  the  temperature  of  the  steam  below 


THE  REAL  STEAM  ENGINE  83 

that  of  the  walls,  and  then  the  direction  of  heat  transfer 
is  reversed,  the  hot  walls  giving  heat  to  the  cooler  steam 
at  an  increasingly  rapid  rate.  This  heat  causes  re-evapora- 
tion of  some  of  the  water  formed  before  and  thus  tends  to 
increase  the  volume  occupied  by  the  material  in  the  cylinder, 
with  the  result  that  the  lower  part  of  the  expansion  curve 
be  approaches  and  generally  crosses  the  curve  which  would 
have  been  attained  by  adiabatic  expansion  in  non-conduct- 
ing apparatus. 

In  many  real  engines  the  re-evaporation  is  so  great  that 
the  steam  is  entirely  dried  and  sometimes  superheated 
before  the  exhaust  valve  opens. 

The  exhaust  valves  of  steam  engines  are  always  opened 
before  the  piston  reaches  the  end  of  the  stroke,  as  it  is  found 
necessary  to  do  this  if  excessive  losses  are  not  to  occur 
due  to  the  difficulty  of  forcing  the  large  volume  of  low- 
pressure  steam  through  the  exhaust  passages.  When  opened 
early  enough,  the  steam  flows  out  in  such  quantity  before 
the  end  of  the  stroke  that  the  "  back  pressure  "  during  the 
return  or  exhaust  stroke  is  only  a  pound  or  two  above  that 
of  the  space  into  which  the  engine  is  exhausting. 

During  all  of  the  exhaust  period,  the  steam  is  probably 
at  a  lower  temperature  than  the  walls  to  which  it  is  exposed 
and  re-evaporation  probably  continues  in  most  cases  until 
the  closure  of  the  exhaust  valve.  It  seems  probable  that 
the  steam  retained  in  the  cylinder  after  the  closure  of  the 
exhaust  valve  is  approximately  dry,  but  little  is  really 
known  regarding  the  quality  of  the  clearance  steam. 

The  rise  of  pressure  during  compression  has  two  bene- 
ficial effects:  It  helps  to  bring  the  moving  parts  to  rest  grad- 
ually, and  it  raises  the  temperature  of  the  clearance  steam 
and  of  the  walls  of  the  clearance  space  to  values  nearer 
that  of  the  entering  steam. 

Remembering  that  area  on  a  PF-diagram  represents 
work,  it  is  easily  seen  that  throttling  losses  and  rounding 
of  corners  due  to  slow  valve  action  (which  cause  a  loss  of 


84  STEAM  POWER 

diagram  area)  result  in  a  loss  of  work.  The  fact  that  conden- 
sation also  causes  a  great  loss  is  easily  shown.  A  given 
quantity  of  steam  entering  the  engine  with  its  supply  of 
heat  can,  in  the  ideal  case,  do  a  certain  amount  of  work  at 
the  expense  of  that  heat.  In  the  real  case  part  of  the  heat 
is  stored  in  the  walls  during  the  early  part  of  the  cycle, 
so  that  it  is  not  available  for  the  doing  of  work  and  is  removed 
from  the  walls  and  carried  out  into  the  exhaust  as  unutilized 
heat  during  the  later  part  of  the  cycle.  The  phenomenon 
can  be  pictured  by  imagining  the  steam  as  dropping  some 
of  its  heat  into  a  pocket  in  the  walls  of  the  cylinder  when 
entering  the  engine  and  then  picking  it  up  again  and  carrying 
it  out  when  leaving,  so  that  the  next  charge  of  steam  will 
have  to  fill  the  pocket  again. 

The  net  result  of  condensation  and  re-evaporation 
is  the  obtaining  of  less  work  from  a  given  quantity  of  steam 
than  should  be  obtained,  or  the  use  of  more  steam  than 
theoretically  necessary  for  a  given  quantity  of  work.  This 
effect  is  shown  graphically  by  the  two  adiabatic  expansion 
lines  of  Fig.  34. 

The  initial  condensation  in  real  engines  which  are  sup- 
plied with  saturated  steam  generally  amounts  to  from  20  to 
50  per  cent  of  all  the  steam  supplied,  so  that  it  is  evident 
that  anything  which  will  prevent  part  or  all  of  this  loss  should 
do  much  to  improve  the  steam  consumption  of  engines. 
This  subject  will  be  discussed  in  more  detail  in  later  para- 
graphs and  various  methods  of  decreasing  losses  from  this 
source  will  be  considered. 

54.  Clearance.  The  term  clearance  is  used  in  a  two- 
fold sense;  (a)  to  refer  to  mechanical  clearance  or  the 
linear  -  distance  between  the  two  nearest  points  of  cylinder 
head  and  piston  face  when  the  piston  is  at  the  end  of  its 
stroke,  and  (b)  to  refer  to  volumetric  clearance  or  the 
volume  enclosed  between  the  face  of  the  valve,  the  cylinder 
head  and  the  face  of  the  piston  when  the  latter  is  at  the 
end  of  its  stroke. 


THE  REAL  STEAM  ENGINE 


85 


yalve 


The  former  is  generally  given  in  inches  and  varies 
from  a  very  small  fraction  of  an  inch  in  the  best  engines 
to  an  inch  or  more  in  cheap  and  in  poorly  designed  engines. 
It  is  indicated  by  a  in  Fig.  35. 

The  volumetric  clearance  is  expressed  as  a  percentage 
of  the   piston  displacement  or    steamPort  steam  Chest 
volume  swept  through  by  the 
piston.     It   varies    from  2   per 
cent  or  less  in  the  best  engines 
to  as  high   as   15  per  cent  in 
the  cheaper  and  less  economical 
models.     It  is  made  up  of  the 
parts  designated  by  c  in  Fig.  35. 

55.  Cushion  Steam  and  Cyl 
inder  Feed.      It  is  customary 
to  imagine  the  steam  operating 
within  an   engine    cylinder   to  FlG"  35. -Mechanical  and  Volu- 

.  ,     ,.  ,  7  .  metric  Clearances, 

consist  oi  two  parts,  the  cushion 

steam  and  the  cylinder  feed.  The  former  is  that  part  of 
the  total  which  is  contained  in  the  clearance  space  before 
the  admission  valve  opens  and  serves  to  cushion  the 
reciprocating  parts  of  the  engine.  The  cylinder  feed 
is  the  steam  which  enters  through  the  valve  for  each 
cycle. 

If  the  same  cycle  is  produced  time  after  time  so  that  all 
temperature  effects  are  repeated  at  regular  intervals  and 
so  that  all  events  occur  at  the  same  points  in  successive 
cycles,  the  quantity  of  steam  retained  in  the  clearance 
volume  will  be  the  same  for  successive  cycles.  It  is 
impossible  to  measure  the  quantity  of  this  steam  directly 
and  indirect  methods  are  therefore  adopted  for  that 
purpose. 

It  is  often  assumed  that  the  steam  is  dry  and  satu- 
rated when  compression  begins,  as  at  the  point  e  in  Fig. 
34.  With  this  assumption,  the  weight  of  cushion  steam 
can  be  determined  by  dividing  the  volume  occupied,  that  is, 


86  STEAM  POWER 

Ve,  by  the  volume  occupied  by  one  pound  of  dry  saturated 
steam  at  the  same  pressure.     Thus, 

Cushion  steam  =  5-  —^  Ibs.     .     (29) 

Sp.vol.  at  pressure  Pe 

The  weight  of  cylinder  feed  can  be  very  accurately 
determined  by  condensing  and  weighing  the  steam  leaving 
the  engine  in  a  given  time  and  dividing  by  the  number  of 
cycles  performed  during  the  same  period.  It  can  also  be 
determined  by  metering  the  steam  entering  the  engine 
or  by  measuring  the  water  fed  to  a  boiler  supplying  only  the 
engine  in  question.  An  approximate  determination  of  the 
quantity  of  the  cylinder  feed  can  also  be  made  directly  from 
an  indicator  diagram  by  determining  what  is  known  as  the 
diagram  water  rate.  This  will  be  considered  in  detail  at 
a  later  point. 

When  cushion-steam  and  cylinder-feed  have  both  been 
determined,  the  weight  of  steam  contained  in  the  cylinder 
between  cut-off  and  release  can  be  found  by  adding  the  two 
quantities.  Thus, 

W  =  Wf+WK, (30) 

in  which 

W  =  total  weight  of  steam  expanding  in  cylinder  per  cycle ; 
Wf=  weight  of  cylinder  feed  per  cycle;    and 
WK  =  weight  of  cushion  steam  per  cycle. 

The  volume  which  the  mixture  would  occupy  if  dry 
and  saturated  at  any  given  pressure  can  be  determined  by 
multiplying  W  the  total  weight  by  the  specific  volume  for 
that  particular  pressure. 

56.  Determination  of  Initial  Condensation.  The  loss 
due  to  initial  condensation  is  so  important  that  it  is  cus- 
tomary to  determine  the  amount  of  this  loss  when  studying 
engines.  This  can  be  done  with  fair  accuracy  by  means 
of  the  indicator  diagram. 


THE  REAL  STEAM  ENGINE 


87 


To  make  such  a  study  it  is  necessary  to  know  the  total 
weight  of  material  in  the  engine  cylinder  at  the  point  of 
cut-off.  This  weight  may  be  determined  by  any  of  the 
methods  just  given.  With  the  weight  known,  the  volumes 
which  this  material  should  occupy  at  different  pressures 
if  dry  and  saturated  can  be  determined  by  jnultiplying  by 
the  specific  volumes  at  the  various  pressures.  Plotting 
these  points  on  a  PF-diagram  and  connecting  them  will 
give  a  saturation  curve  for  the  material  in  the  cylinder  such 
as  the  curve  shown  in  Fig.  36. 

By  drawing  this  curve  on  the  indicator  diagram  ob- 
tained from  the  engine  and  then  comparing  distances 
such  as  ab  and  ac  as  explained  in  section  26  of  Chapter  III 


FIG.  36. 


FIG.  37. 


the  quality  of  the  steam  within  the  cylinder  at  all  pressures 
between  cut-off  and  release  can  be  determined.  The  weight 
of  initial  condensation  (up  to  the  point  of  cut-off)  must 
be  the  total  weight  of  water  shown  as  existing  within  the 
cylinder  at  that  point  minus  any  water  brought  in  by  the 
steam  if  it  was  not  dry  when  entering  the  engine. 

Should  the  saturation  curve  cross  the  real  expansion 
curve,  as  shown  in  Fig.  37,  it  indicates  that  the  steam  oc- 
cupies volumes  greater  than  the  specific  volumes  toward 
the  end  of  the  expansion;  the  steam  within  the  cylinder 
must  therefore  be  superheated  during  this  part  of  the 
cycle. 

Many  formulas  have  been  devised  for  giving  the  quan- 
tity of  initial  condensation.  They  are  all  based  upon  the 
results  of  experiment  and  generally  only  give  reliable 


88  STEAM  POWER 

values  for  cases  similar  to  those  used  in  developing  them. 
One  formula  of  this  sort  which  has  been  very  widely  tested 
and  been  found  to  give  reliable  results  within  its  field 
of  applicability  is  that  devised  by  Robert  C.  H.  Heck  and 
explained  in  his  books  on  the  steam  engine.  The  formula  is 


-, (31) 

VA    v  Pe 

in  which 

m  =  the  fraction  representing  initial  condensation;  for 
ordinary  cases  it  is  the  fraction  of  the  cylinder 
feed  which  is  condensed  during  admission,  but  when 
compression  is  very  high  and  when  great  weights 
of  steam  are  retained  in  the  clearance  it  is  the  frac- 
tion of  all  the  material  within  the  cylinder  which 
exists  in  liquid  form  at  the  time  of  cut-off; 
£  =  a  coefficient,  which  varies  between  0.25  and  0.30  with 
type  of  engine.  May  be  assumed  at  0.27  for  average 
work; 

N  =  engine  speed  in  revolutions  per  minute  (R.P.M.); 
s  =  a  constant  for  any  engine,  equal  to  nominal  surface  in 
square  feet  divided  by  nominal  volume  in  cubic 
feet.  The  nominal  surface  is  the  area  of  the  inner 
walls  and  the  ends  of  a  cylinder  with  diameter  equal 
to  the  internal  diameter  of  the  cylinder  and  with 
a  length  equal  to  the  stroke  of  the  engine.  The 
nominal  volume  is  the  cubic  contents  of  such  a 
cylinder; 

s  =  — (  2-^— }-4  )  in  which  D  and  S  represent  diameter  and 


stroke  of  engine  in  inches; 
=  a  temperature  function   obtained   from   Table   II   as 

there  indicated; 
=  the  absolute  pressure  in  cylinder  in  pounds  per  square 

inch  just  after  completion  of  cut-off; 


THE  REAL  STEAM  ENGINE 


89 


e  =  cut-off  ratio,  that  is,  ratio  of  cylinder  volume  opened 
up  by  time  cut-off  has  just  been  completed  to  the 
total  piston  displacement. 

TABLE  II 
FOR  FINDING  VALUES  OF  0  FOR  USE  IN  HECK  FORMULA 

=  k\—  kz  when  ki  and  fa  are  chosen  from  table  for  highest  and  lowest  pressures 
existing  in  cylinder 


p 

k 

p 

k 

p 

k 

p 

k 

p 

k 

p 

k 

1 

175 

15 

210 

50 

269.5 

90 

321.5 

160 

389 

230 

441 

2 

179 

20 

220 

55 

277 

100 

332.5 

170 

397 

240 

447.5 

3 

183 

25 

229 

60 

284 

110 

343 

180 

405 

250 

454 

4 

186 

30 

238 

65 

291 

120 

353 

190 

413 

260 

460.5 

6 

191 

35 

246 

70 

297.5 

130 

362.5 

200 

420 

270 

467 

8 

196 

40 

254 

75 

304 

140 

371.5 

210 

427 

280 

473 

10 

200 

45 

262 

80 

310 

150 

380.5 

220 

434 

290 

479 

57.  Methods    of    Decreasing    Cylinder    Condensation. 

Before  discussing  methods  of  decreasing  the  loss  due  to  cylin- 
der condensation  it  will  be  well  to  consider  what  things  may 
be  expected  to  determine  the  extent  of  such  loss.  The 
.  condensation  is  due  directly  to  the  transfer  of  heat  from 
one  body  to  another  at  lower  temperature,  and  anything 
which  tends  to  increase  the  total  amount  of  heat  thus  trans- 
ferred will  increase  the  total  condensation. 

It  is  therefore  evident  that  the  ratio  of  steam  condensed 
to  steam  supplied  will  be  greatest  when: 

(a)  The  time  of  contact  is  greatest; 

(6)  The  ratio  of  surface  exposed  to  volume  enclosed  is 
greatest,  and 

(c)  The  temperature  difference  is  greatest. 

The  time  of  contact  can  be  controlled  to  a  certain 
extent  by  controlling  the  speed  of  the  engine  and,  with 
other  things  equal,  the  higher  the  speed  the  lower  should 
be  the  condensation. 

The  ratio  of  surface  exposed  to  steam  to  the  volume 
occupied  by  steam  has  a  great  influence  on  the  amount  of 


90  STEAM  POWER 

condensation  which  occurs.  The  surface  of  the  clearance 
space,  including  the  interior  surfaces  of  all  ports  or  passages 
leading  to  the  valves,  seems  to  have  the  greatest  influence, 
and  the  clearance  space  which  is  most  nearly  a  short  cylinder 
without  connected  passages  may  be  expected  to  give  the 
least  h  itial  condensation. 

The  size  of  the  engine  is  also  important  in  this  connec- 
tion. The  area  exposed  does  not  increase  as  rapidly  as 
does  the  volume  inclosed  when  the  diameter  of  a  cylinder  is 
increased,  and  therefore  large  cylinders  give  smaller  ratio 
of  surface  to  volume  and  therefore  a  smaller  percentage 
of  steam  condensed.  Large  engines  thus  have  a  decided 
advantage  over  small  engines. 

The  shape  of  the  cylinder  also  has  an  effect.  The 
longer  the  cylinder  with  respect  to  its  diameter  the  more 
favorable  its  performance. 

The  point  at  which  cut-off  occurs  is  also  intimately 
connected  with  the  condensation  loss.  In  a  given  cylinder 
with  a  given  clearance  the  total  condensation  within  the 
clearance  space  may  be  assumed  practically  constant  if 
speed  and  temperature  remain  about  the  same.  But  if 
the  cut-off  is  made  later  larger  quantities  of  steam  are 
admitted  per  stroke,  and  hence  the  fraction  of  the  total 
cylinder  feed  which  is  condensed  decreases. 

The  temperature  differences  depend  on  upper  and 
lower  pressures,  that  is,  on  the  pressure  range.  The  inner 
surfaces  of  the  walls  follow  as  rapidly  as  possible  the  tem- 
perature changes  of  the  steam  within  them.  Thus  their 
average  temperature  is  somewhere  between  the  upper  and 
lower  temperatures  of  the  steam.  If  now,  with  a  given 
upper  steam  pressure  and  therefore  temperature,  the  lower 
pressure  be  reduced,  the  average  wall  temperature  also  will 
be  reduced,  and  therefore  the  differences  between  the 
temperature  of  the  entering  steam  and  the  average  tem- 
perature of  the  walls  will  be  increased  with  a  resulting  in- 
crease in  condensation  loss. 


THE  REAL  STEAM  ENGINE  91 

The  methods  of  decreasing  this  loss  can  now  be  con- 
sidered. They  are  given  below  under  separate  heads 
with  brief  explanation  when  necessary. 

(a)  Clearance  spaces  should  be  properly  designed  so 
that  the  minimum  surface  is  exposed. 

(6)  The  propoitions  of  cylinder  (diameter  and  stroke) 
and  the  speed  of  tlie  engine  should  be  so  chosen  that  the 
condensation  loss  is  reduced  to  a  minimum. 

(c)  The  engine  should  be  so  proportioned  that  when 
delivering  its  rated  power  the  cut-off  occurs  at  such  a  point 
as  to  make  the  percentage  of  cylinder  condensation  the 
minimum  consistent  with  other  requirements. 

(d)  The  cylinder  should  be  surrounded  by  spaces  filled 
with  air  or  by  materials  which  are  poor  conductors  of  heat 
so  as  to  decrease  loss  by  radiation,  because  all  heat  lost  in  this 
way  must  be  supplied  by  the  condensation  of  steam  within 
the  cylinder.     Such  metallic  parts  as  cannot  be  " lagged" 
in  this  way  should  be  polished  because  polished  surfaces 
radiate  less  heat  than  dull  surfaces  under  like  conditions. 

(e)  The  cylinder  may  be  surrounded  by  a  steam  jacket, 
that  is,  a  space  filled  with  steam  similar  to  that  supplied 
the  cylinder.     The  use  of  such  a  jacket  sometimes  results 
in  a  considerable  saving  and  at  other  times  in  a  great  loss. 
The  cylinder  proportions,  speed  and  pressure  range  seem 
to  be  the  determining  factors,  and  most  long-stroke  cylin- 
ders operating  at  low  rotative  speed  and  with  small  pressure 
ranges  are  jacketed. 

(/)  The  engine  may  be  compounded,  that  is,  the  expan- 
sion of  the  steam  may  be  made  to  occur  in  two  or  more 
cylinders  taking  steam  in  series.  This  results  in  decreas- 
ing the  pressure  range  in  each  of  the  cylinders  and  effects 
a  decided  saving  under  proper  conditions.  Compounding 
will  be  considered  in  detail  in  a  later  chapter. 

(g)  The  engine  may  be  supplied  with  superheated  steam. 
If  the  steam  is  sufficiently  superheated  it  can  give  up  part 
or  all  of  its  superheat  to  heat  the  cylinder  walls,  and  thus  no 


92  STEAM  POWER 

condensation  need  occur.  Heat  interchanges  between 
metal  and  superheated  steam  also  appear  to  be  less  rapid 
than  is  the  case  when  the  steam  contains  water,  so  that  a 
saving  results  from  this  source  also. 

Tests  made  with  saturated  and  with  superheated  steam 
indicate  that  from  7°  to  10°  of  superheat  are  generally 
required  to  prevent  1  per  cent,  of  initial  condensation. 
Results  differ  greatly  with  the  character  of  the  engine,  with 
its  economy  on  saturated  steam,  with  its  valve  gear,  etc. 
Superheats  of  from  25°  to  50°  can  generally  be  used  with 
any  well-designed  engine,  but  higher  temperatures  usually 
require  specially  constructed  engines.  Superheats  as  high 
as  150°  F.  above  saturation  temperature  are  now  quite 
common,  and  there  seems  to  be  a  tendency  to  consider  a 
value  between  150°  and  200°  as  the  highest  that  is  com- 
mercially advisable  under  ordinary  conditions. 

58.  Classification  of  Steam  Engines.  Steam  engines, 
are  classified  on  many  different  systems,  the  one  used  in 
any  particular  case  being  determined  largely  by  circum- 
stances. The  principal  methods  of  classification  are  indi- 
cated in  the  following  schedule : 

CLASSIFICATION  OF  STEAM  ENGINES 

fLow  speed 

On  the  basis  of  rotative  speed  \  Medium  speed 

[High  speed 

On  the  basis  of  ratio  of  stroke  to  diameter  \ 

Short  stroke 


On  the  basis  of 


D-slide  valve 

Balanced  slide  valve 
Slide  valve       ,T  ,,.  ... 

Multiported  slide  valve 

Piston  valve 


valve  gear 

„    ..  I  Drop  cut-off 

Corliss  valve  i  „     ...     . 

[Positively  operated 

Poppet  valve 


THE  REAL  STEAM  ENGINE 


93 


[Vertical 
On  the  basis  of  position  of  longitudinal  axis  I  Inclined 

[Horizontal 
Single  expansion  or 
Simple  engine 

f  Compound  expansion 
|  Triple  expansion 
I  Quadruple  expansion 


On  the  basis  of 
number  of  cyl- 
inders in  which 
steam  expands 


Multi-expansion 
engine 


On  the  basis  of  cylinder  arrangement 


On  the  basis  of  use 


Single  cylinder 
Tandem  compound 
Cross  compound 
Duplex 

Stationary  engines 

Portable  engines 

Locomotive  engines 

Marine  engines 

Hoisting  engines 


59.  Rotative  Speeds  and  Piston  Speeds.  High-speed 
engines  operate  at  a  comparatively  high  rotative  speed  and 
are  characterized  by  a  short  stroke  in  comparison  with  the 
diameter  of  the  cylinder,  the  stroke  generally  being  equal 
to,  or  less  than,  the  diameter.  The  piston  speed,  by  which 
is  meant  the  feet  travelled  by  the  piston  per  minute,  generally 
falls  between  500  and  700. 

It  is  not  considered  advisible  to  allow  piston  speeds  of 
stationary  steam  engines  to  exceed  about  750  feet  per 
minute  for  ordinary  constructions  and  the  great  majority 
of  engines  give  much  lower  values.  The  piston  speed  will 
obviously  be  given  by  the  formula 


in  which 


=  2LN, 


(32) 


piston  speed  in  feet  per  minute; 
stroke  in  feet  ;    and 


, 

N  =  revolutions  per  minute, 


94 


STEAM  POWER 


and  it  is  evident  from  this  formula  that  as  the  rotative 
speed  is  increased  the  piston  speed  will  increase  unless 
the  length  of  stroke  is  proportionately  decreased.  As 
a  result,  high-speed  engines  have  short  strokes  in  com- 


\ 

o/O 

350 
325 
300 

275 
250 
225 
800  o    200 
700  |    175 
600  rg    1-50 

500^   125 

1 
400  £   100 

\ 

H 

IGH 

SP 

EE 

D 

EN 

Gl 

NE 

S 

\ 

/ 

\ 

/ 

X; 

\ 

/ 

> 

/ 

\ 

x> 

sv 

/ 

\ 

/ 

\ 

/ 

\ 

/ 

4 

^ 

N 

/ 

\ 

Ac> 

/ 

x 

f 

/ 

sv 

,  t 

|»C 

id 

N 

^ 

^ 

*~~- 

V0< 

'**" 

^ 

s* 

( 

•"3* 

V 

^y 

#. 

"X1 

8         10        12        14        16        18        20        22        24        26 
Diameter  of  Cylinder  (Inches) 

FIG.  38. — Proportions  of  High  Speed  Engines. 


parison  with  their  cylinder  diameters  and  slow-speed 
engines  have  long  strokes. 

The  characteristic  relations  between  cylinder  diameter 
and  stroke,  rotative  speed  and  piston  speeds  of  high-speed 
engines  are  given  in  Fig.  38. 

High-speed    engines    are    generally    fitted    with    some 


THE  REAL  STEAM  ENGINE 

R.  P.  M. 


95 


Stroke  (L)  in  Inches    ] 


96  STEAM  POWER 

form  of  balanced  slide  valve,  and  are  controlled  by  what 
is  called  a  shaft  governor.  They  are  very  compact,  having 
small  weight  and  occupying  small  space  in  comparison  with 
the  power  developed. 

Slow-speed  engines  are,  in  general,  the  most  economical 
and  are  characterized  by  low  rotative  speed,  long  stroke, 
and  elaborate  valve  gear.  The  weight  per  horse-power 
is  high,  and  they  generally  occupy  a  great  deal  of  space. 
The  Corliss  engine  is  the  best  known  and  most  widely  built 
engine  of  this  type. 

The  characteristic  relations  of  cylinder  diameter  to 
stroke,  rotative  speed  and  piston  speed  for  slow-speed 
engines  are  given  in  Fig.  39. 

Medium-speed  engines  generally  operate  at  rotative 
speeds  between  150  and  250  R.P.M.  They  are  generally 
fitted  with  the  better  forms  of  multiported  and  balanced 
slide  valves,  with  poppet  valves,  or  with  a  positively  operated 
Corliss  type  of  valve. 

60.  The  Simple  D-slide  Valve  Engine.  The  simplest 
and  cheapest  type  of  reciprocating  steam  engine  manu- 
factured is  shown  in  part  section  in  Fig.  40  with  the  prin- 
cipal parts  labelled.  The  cylinder,  piston,  steam  chest 
and  valve  are  sectioned  in  order  to  show  the  internal  con- 
struction. 

This  engine,  like  most  steam  engines,  is  double  acting, 
that  is,  a  cycle  is  produced  on  each  side  of  the  piston  during 
every  revolution.  Steam  is  admitted  and  expanded  on  one 
side  of  the  piston  while  steam  is  being  exhausted  on  the  other 
side.  The  control  of  admission  and  exhaust  is  effected  by 
the  slide  valve  and  will  be  considered  in  detail  in  later  sec- 
tions. 

The  mechanical  energy  made  available  by  the  steam 
operating  in  an  engine  cylinder  is  not  developed  at  a  uniform 
or  constant  rate,  but  fluctuates,  during  each  revolution, 
above  and  below  the  amount  required  to  overcome  the 
constant  resistance  at  the  shaft  due  to  the  work  the  engine 


THE  REAL  STEAM  ENGINE  97 

is  doing.  If  no  provision  were  made  to  prevent  it,  this 
would  result  in  a  very  variable  rate  of  rotation  during 
each  revolution.  When  the  energy  made  available  was  in 
excess  of  the  demand  it  would  be  used  in  accelerating 
the  moving  parts  of  the  engine  and  the  speed  of  the  latter 
would  increase.  The  reverse  would  occur  when  the  supply 
did  not  equal  the  demand. 

The   fly-wheel   is   used  to   prevent  violent  fluctuations 


cabined  Flywheel 
and  Beltwheel 


FIG.  40. — Simple  D-slide  Valve  Engine. 

of  this  kind.  It  is  made  with  a  comparatively  heavy  rim 
and  a  great  deal  of  energy  must  be  supplied  to  accelerate 
it  to  any  appreciable  extent  in  a  short  time.  Similarly 
it  can  give  out  a  great  deal  of  energy  when  slowing  down. 
The  fly-wheel  therefore  serves  as  a  sort  of  reservoir  in  which 
excess  energy  can  be  stored  temporarily  and  from  which 
it  can  later  be  withdrawn  when  a  deficiency  exists.  The 
fly-wheel  thus  acts  as  a  damper  to  variation  of  rotative 


98  STEAM  POWER 

speed  during  each  revolution,  minimizing  but  not  entirely 
eliminating  such  variation.  It  may  also  serve  as  a  belt 
wheel,  as  shown  in  the  illustration. 

The  governor  controls  the  steam  supply  to  the  cylinder 
in  such  a  way  that  enough  heat  will  be  supplied  to  make 
available  the  power  demanded  at  the  shaft.  Were  more 
supplied  the  excess  would  be  absorbed  by  the  moving 
parts  and  the  engine  speed  would  increase,  were  less  supplied 
the  engine  speed  would  decrease. 

61.  Engine  Nomenclature.  The  meanings  of  several 
terms  used  in  describing  engines  are  not  self-evident,  their 


Belt  Backward 


^      Belt  Forward 


FIG.  41. — Engine  Nomenclature. 

definitions  depending  merely  on  accepted  usage.  Some 
of  these  terms  and  their  meanings  are  illustrated  in  Fig.  41. 

The  crank  end  of  a  horizontal  engine  is  called  the  front 
of  the  engine,  so  that  the  cylinder  head  nearest  the  crank 
is  called  the  front  head  and  the  stroke  of  the  piston  toward 
the  crank  is  known  as  the  forward  stroke.  The  forward 
stroke  of  the  piston  is  also  spoken  of  as  the  outstroke,  par- 
ticularly in  connection  with  single  acting  engines.  The 
stroke  away  from  the  crank  is  correspondingly  designated 
as  the  return  or  the  instroke. 

62.  Principal  Parts  of  Engines.  The  parts  of  engines 
may  be  roughly  divided  into  stationary  and  moving,  such  as 
frame,  cylinder,  cylinder  and  valve  chest  covers,  etc.,  which 


THE  REAL  STEAM  ENGINE 


99 


are  stationary,  and  piston,  piston  rod,  crosshead,  fly-wheel, 
etc.,  which  are  all  moving  parts  when  the  engine  is  in  opera- 
tion. The  moving  parts  are  often  divided  into  reciprocal 


FIG.  42. — Frame  for  Small  Vertical  Engine. 

ing  and  rotating  parts.  Thus  the  piston  and  all  connected 
parts  through  and  including  the  crosshead,  and  the  valve 
and  many  connected  parts  in  the  case  of  slide  valve  engines, 


Jaws  for 

main  bearings 


FIG.  43. — Frame  for  Medium  Speed  Center  Crank  Engine. 


all  reciprocate  when  the  engine  is  in  operation.  The 
shaft,  fly-wheel,  eccentric  sheaves  and  governor  constitute 
the  principal  rotating  parts. 


100 


STEAM  POWER 


Some  engines  also  have  oscillating  parts,  such  as  the 
valves  in  Corliss  types,  which  rock  back  and  forth  in  the 
arc  of  a  circle,  and  the  rocker  arms  in  various  forms  of 
valve  gear,  these  arms  rocking  through  a  short  arc  about 
a  fixed  pin  near  one  end. 

The  principal  parts  and  their  functions  are  briefly  con- 
sidered in  the  following  paragraphs: 

(a)  The  Frame.  The  frame  of  the  engine,  sometimes 
known  as  the  bed,  serves  to  support  the  other  parts,  to  tie 


Cylinder  bolted 
to  finished 
urface  here. 


Bored  cross 
head  guides 


FIG.  44. — Frame  for  Slow  S^eed  Engine  of  Corliss  Type;    Side  or 
Overhung  Crank. 

them  together  in  their  proper  relations  and  to  fasten  the 
whole  structure  to  whatever  foundation  is  used.  The  cross- 
head  guides  and  the  seats  for  the  main  bearings  are  incor- 
porated in  the  frame. 

The  frame  is  commonly  made  of  cast  iron  in  the  form 
of  a  hollow  box  which  is  properly  ribbed  to  give  the  neces- 
sary stiffness. 

Examples  of  frames  are  shown  in  Figs.  42,  43  and  44. 


THE  REAL  STEAM  ENGINE 


101 


(6)  The  Cylinder  and  Steam  Chest.  The  cylinder  and 
steam  chest  are  generally  incorporated  in  the  same  casting, 
and  surfaces  of  covered  cavities  in  this  casting  are  finished 


Steam  Pipe 


Steam  Chest' 
Cover 


m  Chest 


FlG.  45. 


.Steam  Chest  Cover 


to  form  the  cylinder  in  which  the  piston  operates  and  the 
seat  or  seats  upon  which  valves  rest  and  move. 

The  cylinder  may  be  single  .walled  with  flanges  on  the 
end  to  receive  the  cylinder  head,  as  illustrated  in  Fig.  40 


102 


STEAM  POWER 


(plain  D-slide  valve),  in  which  case  a  thin  sheet-metal 
jacket  is  fastened  around  it  and  the  space  between  filled 
with  heat-insulating  material.  Or,  the  cylinder  may  be 
cast  with  double  walls,  the.  space  between  the  two  being 
used  as  an  air  jacket  or  as  a  steam  jacket. 

Some  cylinders  are  fitted  with  a  liner,  which  is  a  plain 
cylinder  pressed  into  place  within  the  cylinder  casting 
and  forming  the  bore  of  the  working  cylinder.  This  prac- 


FIG.  47. — Section  of  Atlas  Medium  Speed  Engine,  Showing  Balanced 

Slide  Valve. 

tice  is  common  on  the  larger  types,  the  liner  being  used  so 
that  when  wear  has  occurred  it  can  be  replaced  cheaply, 
instead  of  it  being  necessary  to  rebore  or  even  replace 
the  main  casting. 

Examples  of  cylinder  construction  are  shown  in  Figs. 
45,  46,  47,  48,  49  and  50. 

(c)  The  Piston.  The  function  of  the  piston  is  two- 
fold. It  must  prevent  the  leakage  of  steam  by  it  from  one 
end  of  the  cylinder  to  the  other,  and  it  must  receive  the 


THE  REAL  STEAM  ENGINE 


103 


I 


O 

f 


104 


STEAM  POWER 


pressures  exerted  by  the  steam  and  transmit  them  to  the 
other  parts  of  the  mechanism  as  it  moves. 


Chambers  for 

Stea.u  Valves 

(admission  valves) 


Chambers  for 
Exhaust  Valves 


FIG.  49. — Corliss  Cylinder  Casting. 
-  Steam  Con  nectlon 


FIG.  50. — Corliss  Cylinder  with  Lagging  in  Place. 

Leakage  of  steam  is  prevented  by  the  use  of  piston 
rings,  which  are  metal  rings  fitted  into  grooves  in  the  circular 
surface  of  the  piston  and  pressed  out  against  the  cylinder 


THE  REAL  STEAM  ENGINE 

Pistons 


105 


FIG.  51. 


(a)  (6) 

FIG,  52. 


'Bull  Rings 

FIG.  53. — Built-up  Piston  Used  in  Large  Engines. 


106 


STEAM  POWER 


walls  by  spring  action.  They  may  be  made  of  one  piece 
of  metal  turned  into  a  ring  of  slightly  larger  diameter  than 
the  cylinder,  cut  through  and  sprung  into  place,  or  they  may 
be  made  in  pieces  as  shown  in  Fig.  51,  and  pressed  out 
against  the  wall  by  small  helical  or  leaf  springs. 

.  The  piston  itself  may  consist  of  a  solid  disk  of  metal 
fitted  with  a  hub  and  a  short  cylindrical  part  with  grooves 
for  the  rings,  as  shown  in  Fig.  52  (a)  and  (6) ,  or  it  may  be 
an  elaborate  built-up  structure,  as  shown  in  Fig.  53. 


FIG.  54.— Crosshead  and  Pin. 

(d)  The  Piston  Rod.  The  piston  rod  is  a  plain  circular 
steel  rod  fitted  with  such  shoulders  and  threads  at  the  ends 
as  are  necessary  for  the  fastening  of  the  piston  and  the  cross- 
head.  Examples  of  such  fastenings  are  given  in  Figs. 
51,  52,  53  and  55. 

In  large  horizontal  engines  the  piston  rod  sometimes 
extends  through  the  piston  and  rear  cylinder  head,  and 
the  rear  end  is  then  supported  by  a  small  auxiliary  cross- 
head.  The  extension  of  the  rod  is  known  as  the  tail  rod 
and  the  auxiliary  crosshead  as  the  tail  rod  crosshead. 


THE  REAL  STEAM  ENGINE 


107 


Such  constructions  are  used  when  the  weight  of  the  piston 
is  so  great  that  it  would  cause  serious  cylinder  wear  if  not 
supported  more  perfectly  than  is  possible  with  the  ordinary 
overhung  arrangement. 


FIG,  55. — Single  Slipper  Crosshead. 


FIG.  56. — Crosshead  with  Adjustable  Slippers. 

(e)  The  Crosshead  and  Guides.  The  crosshead  and 
guides  are  used  for  the  purpose  of  supporting  the  piston 
and  its  rod  and  guiding  them  in  a  straight  line.  The 
crosshead  also  serves  to  connect  the  piston  rod  and  the 
connecting  rod  through  which  the  forces  are  transmitted 
to  the  crank  pin. 


108 


STEAM  POWER 


Crossheads  are  generally  cast  in  the  form  of  imperfectly 
shaped  boxes  and  carry  slippers  which  are  faced  with  anti- 
friction metal  where  they  come  in  contact  with  the  guides. 
The  slippers  may  be  flat  and  operate  on  planed  guides, 
as  shown  in  Fig.  40,  or  they  may  be  turned  and  operate  in 
bored  guides,  as  shown  in  Figs.  48,  54,  55  and  56.  Pro- 
vision is  generally,  though  not  always,  made  for  taking 
up  wear  of  guides  and  slippers  by  setting  the  slippers 
further  out  from  the  body  of  the  casting. 

With  the  type  shown  in  Figs.  40  and  54  this  can  be 


FIG.  57. — Solid  End  Connecting  Rod;  for  Overhung  Cranks  only. 

done  by  the  insertion  of  thin  sheets  of  rnetal  or  paper 
(known  as  shims)  between  the  body  of  the  crosshead  and  the 
slippers.  In  the  type  shown  in  Fig.  56  the  slippers  are 
finished  with  inclined  surfaces  where  they  come  in  contact 
with  the  main  casting,  and  the  adjustment  is  made  by  wedg- 
ing the  slippers  apart  by  the  use  of  the  adjusting  bolts 
shown. 

The  wrist-pin  end  of  the  connecting  rod  enters  the 
crosshead  casting  and  is  held  in  place  by  means  of  the 
wrist  pin,  about  which  it  oscillates  when  the  engine  is  in 
operation . 


THE  REAL  STEAM  ENGINE 


109 


(/)  The  Connecting  Rod.  This  rod  connects  the  re- 
ciprocating crosshead  with  the  rotating  crank  shaft  and 
transmits  the  forces  from  one  to  the  other.  It  consists 
of  a  body  or  shank  and  two  ends  or  heads.  The  ends  may 


FIG.  58. — Connecting  Rod  with  Bolted  Strap  Ends;    May  be  Used 
with  Center  or  Side  Crank  Constructions. 


be  "  closed  "  or  "  solid  "  as  shown  in  Fig.  57;    they  may 
be  made  with  a  strap  bolted  in  place  as  shown  in  Fig.  58; 
or  the  crank-pin  end  may  be  made  of  two  half  boxes  bolted 
together  to  form  a  "  marine  end  "  as  shown  in  Fig.  59. 
The  ends  are   always  made   adjustable   so   that   wear 


,  Crank  End 


Crosshead  End. 


FIG.  59. — Marine  End  Connecting  Rod. 

can  be  taken  up,  thus  preventing  noisy  operation  due  to 
hammering  between  the  ends  and  the  pins  at  times  when 
the  direction  in  which  forces  act  is  reversed.  With  solid 
and  strap  types  this  adjustment  is  generally  made  by 
means  of  wedges  similar  to  those  shown  in  Figs.  57  and  58. 


110 


STEAM  POWER 


With  the  marine  type  shims  are  used  between  the  two 
halves,  and  the  diameter  of  the  hole  formed  by  the  latter 
is  decreased  by  the  removal  of  shims  of  the  required  thick- 
ness. 

(g)  The  Shaft.     The  crank  shaft  itself  is  generally  made 


kPin 


k.Arms 


^-Journal* 

FIG  60. — Crank  Shaft,  Center  Crank. 


Counterweights 
or  Balances 

FIG.  61.— Center  Crank. 


Counterweight 

FIG.  62.— Center  Crank. 

of  steel,  but  the  counterbalances  are.  often  of  cast  iron.  It 
may  be  one  forging  throughout  or  may  be  built  up  by 
shrinking  the  various  parts  together.  Multicrank  shafts 
of  large  size  are  generally  of  built-up  construction,  the  crank 
pins  being  shrunk  into  the  crank  arms  and  the  latter  shrunk 
on  to  the  pieces  of  shaft. 

The  counterbalance  weights  are  used  to  balance  the 


THE  REAL  STEAM  ENGINE  111 

centrifugal  effect  of  the  crank  pin,  part  of  the  crank  arms 
and  part  of  the  connecting  rod,  all  of  which  rotate  off  center. 
In  some  engines  part  of  the  unbalanced  effect  of  the  recip- 
rocating parts  is  also  imperfectly  balanced  by  these  counter- 
balances. 

Various  types  of  shafts  are  shown  in  Figs.  60,  61,  62, 
63,  and  64. 

(h)  Bearings.  Bearings  are  distinguished  as  main  and 
as  outboard  bearings.  Main  bearings  are  those  carried  by 


FIG.  63. — Crank  Shaft  and  Disc,  Overhung  Crank. 


FIG.  64. — Overhung  Crank. 

the  frame  of  the  engine  and  outboard  bearings  are  carried 
by  separate  pedestals  or  by  pedestals  fastened  to  a  plate 
which  is  in  turn  fastened  to  the  frame.  Center-crank 
engines  have  two  main  bearings,  and  side-crank  engines 
only  have  one,  the  other  end  of  the  shaft  being  supported 
by  an  outboard  bearing. 

The  bearings  of  steam  engines  are  generally  formed 
of  babbitt-lined  boxes  carried  within  jaws  machined  in  a 
frame,  or  in  a  separate  pedestal,  and  held  in  place  by  a 
bearing  cap.  The  boxes  are  made  in  two,  three  or  four 
parts  to  allow  for  adjustment  to  compensate  for  wear  and 


112 


STEAM  POWER 


to  give  a  certain  degree  of  flexibility.  Adjustment  for 
wear  is  either  made  by  means  of  wedges  or  by  means  of 
screws  which  force  the  various  parts  of  the  boxes  toward 
the  shaft.  An  example  of  a  three-part  bearing  with  screw 
adjustment  as  used  with  large  side-crank  engines  is  shown 
in  Fig.  65.  The  parts  of  a  three-part  bearing  with  wedge 
adjustment  are  shown  in  Fig.  66. 

Bearings  are  often  lubricated  by  rings  or  chains,  and 
they  are  then  known  as  ring-  or  as  chain-oiling  bearings. 


FIG.  65. — Three  Part  Main  Bearing  with  Screw  Adjustment. 

In  the  ring-oiling  bearing  one  or  more  metal  rings  of  large 
diameter  hang  loosely  on  the  shaft  within  the  bearing  and 
dip  into  an  oil  reservoir  below  the  shaft.  Rotation  of  the 
shaft  causes  the  rings  hanging  on  it  to  rotate  and  they 
carry  oil  up  from  the  reservoir  and  spill  it  out  over  the  shaft 
within  the  bearing.  Chain  bearings  are  similar  except  that 
chains  are  substituted  for  rings. 

(i)  Fly-wheels.  The  function  of  the  fly-wheel  has  al- 
ready been  considered  and  need  not  be  discussed  further. 
The  wheel  is  constructed  with  a  heavy  rim  joined  to  a  hub 


THE  REAL  STEAM  ENGINE 


113 


by  six  or  eight  arms.  In  the  smaller  sizes  the  wheel  may  be 
cast  in  one  piece,  but  best  practice  calls  for  a  split  hub  in 
that  case  to  partly  equalize  certain  casting  strains  which 
result  from  unequal  thicknesses  of  metal  in  different  parts 
of  the  wheel.  Large  wheels  are  -cast  in  two  or  more  parts 
both  for  the  purpose  of  partly  avoiding  casting  strains 
and  for  the  purpose  of  facilitating  handling  and  shipping. 


FIG.  66. — Three  Part  Bearing 
Showing  Wedge  Adjustment. 


FIG.  67. 


A  two-part  wheel  with  the  rim  sections  joined  by 
prisoner  links  shrunk  in  place  and  the  hub  fastened  with 
bolts  is  shown  in  Fig.  67. 

PROBLEMS 

1.  A  given  engine  has  a  piston  displacement  of  3  cu.ft.  and  a 
clearance  volume  of  3%.  Compression  begins  when  85%  of  the 
exhaust  stroke  has  been  completed  and  the  pressure  within  the 
cylinder  at  that  time  is  16  Ibs.  per  square  inch  absolute.  Deter- 
mine the  weight  of  the  cushion  steam  on  the  assumption  that  this 
steam  is  dry  and  saturated  at  the  beginning  of  compression. 


114  STEAM  POWER 

2.  Assume  the  engine  described  in  Prob.  1  to  cut-off  at  |  stroke 
and  with  a  pressure  inside  the  cylinder  equal  to  115  Ibs.  per  square 
inch  absolute.     Find  the  weight  of  cylinder  feed  if  the  quality 
of  the  material  in  the  cylinder  at  the  time  of  cut-off  is  75%. 

3.  Find  the  piston  speed  of  an  engine  with  a  stroke   of  2  ft. 
and  a  rotative  speed  of  150  R.P.M. 

4.  Show  by  means  of  Heck's  formula  that  initial  condensation 
increases  with  pressure  range. 


CHAPTER  VIII 
THE  INDICATOR  DIAGRAM  AND  DERIVED  VALUES 

63.  The  Indicator.  The  ideal  steam  engine  cycle  was 
described  in  Chapter  IV,  and  the  sort  of  diagram  which 
would  be  obtained  from  a  real  engine  was  shown  in  Chapter 


Drum 


Point  Holder 


Cylinde 


FIG.  68. 

VII;    but  the  means  by  which  such  diagrams  are  obtained 
from  operating  engines  was  not  given. 

Indicator  diagrams  showing  the  pressure  and  volume 
changes   experienced   by   steam   in   the   cylinders   of   real 

115 


116 


STEAM  POWER 


engines  are  obtained  by  means  of  an  instrument  known  as 
an  indicator.  The  operation  of  obtaining  such  diagrams  is 
known  as  indicating  the  engine. 

An  external  view  of  one  form  of  indicator  is  shown  in 
Fig.  68  and  a  section  through  the  instrument  is  given  in 
Fig.  69.  The  method  of  connecting  an  indicator  to  the 


-Drum 
Spring 


Piston  Rod 


Connected  to  Engine 
Cylinder 

FlG.  69. 


cylinder  of  a  steam  engine  and  one  method  used  for  driving 
it  are  illustrated  in  Fig.  70. 

The  indicator  is  intended  to  draw  a  diagram  showing 
corresponding  pressures  and  volumes  within  the  engine 
cylinder  and  must,  therefore,  contain  one  part  which  will 
move  in  proportion  to  pressure  variations  and  another  which 
will  move  in  proportion  to  volume  changes.  The  one  may 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES    117 

be  called  the  pressure-measuring  and  the  other  the  volume- 
measuring  device. 

The  pressure-measuring  device  generally  consists  of 
a  piston,  such  as  shown  in  the  figure,  working  with  minimum 
friction  in  a  small  cylinder  and  fitted  with  a  spring  which 
will  resist  what  may  be  called  outward  motion  (upward 
in  the  figure).  The  cylinder  containing  this  piston  is 
coupled  to  a  short  pipe  connected  with  the  clearance  space 
of  the  engine  and,  whenever  the  indicator  cock  in  this 
connection  is  open,  the  steam  acting  on  the  engine  piston 


FIG.  70. — Method  of  Attaching  and  Operating  an  Indicator. 

will  also  act  on  the  indicator  piston.  Steam  of  any  given 
pressure  will  drive  the  indicator  piston  out  against  the 
action  of  the  spring  until  the  pressure  exerted  by  the  spring 
is  equal  to  that  exerted  on  the  face  of  the  piston.  The 
indicator  piston  will  thus  move  out  different  distances  for 
different  pressures,  and,  through  the  piston  rod  and  pencil 
mechanism,  will  move  the  pencil  point  to  various  heights 
corresponding  to  different  steam  pressures.  The  pencil 
mechanism  is  so  arranged  that  the  point  traces  a  straight 
vertical  line  on  the  drum  as  the  indicator  piston  moves  in 
and  out. 

Springs  .are  made  to  certain  definite  scales,  thus  there 


118  STEAM  POWER 

are,  for  instance,  10-lb.,  25-lb.,  50-lb.  and  100-lb.  springs. 
The  number  which  is  known  as  the  scale  of  the  spring 
designates  the  steam  pressure  in  pounds  per  square  inch 
which  is  required  to  move  the  pencil  point  1  inch  against 
the  action  of  such  a  spring.  With  a  100-lb.  spring  in  the 
indicator,  a  steam  pressure  of  50  pounds  per  square  inch 
acting  on  the  indicator  piston  would  drive  the  pencil  up  a 
distance  of  half  an  inch,  a  pressure  of  100  pounds  per  square 
inch  would  give  1  inch  of  motion  and  so  forth. 

The  volume-measuring  device  is  of  an  inferential  kind. 
It  simply  indicates  the  position  attained  by  the  engine 
piston  at  the  time  when  a  given  steam  pressure  existed  in 
the  cylinder  and  the  volume  occupied  by  the  steam  can  be 
calculated  from  piston  position  and  cylinder  dimensions. 
The  position  of  the  piston  is  indicated  by  connecting  the 
cord  wound  around  the  drum  to  some  part  of  the  engine 
which  is  rigidly  connected  to  the  piston.  The  crosshead 
is  commonly  used  for  this  purpose  and,  since  the  motion 
of  this  member  is  generally  much  greater  than  the  circum- 
ference of  the  drum,  it  is  necessary  to  use  a  reducing 
mechanfsm  of  some  sort.  This  mechanism  must  be  very 
accurate,  so  that  it  moves  the  drum  as  nearly  as  possible 
in  proportion  to  the  motion  of  the  engine  piston. 

The  pencil  point  moves  up  and  down  as  the  pressure 
within  the  cylinder  varies,  and  the  drum  rotates  under 
the  point  in  proportion  to  the  motion  of  the  engine  pis- 
ton, so  that  the  combination  of  the  two  motions  brings  the 
pencil  point  to  successive  positions  on  the  drum  which  indi- 
cate successive  corresponding  values  of  steam  pressure  and 
piston  position.  By  mounting  a  piece  of  paper,  known 
as  a  card,  on  the  drum  and  pressing  the  pencil  point  upon 
this  paper,  the  successive  positions  occupied  by  the  pencil 
point  will  be  recorded  in  the  form  of  a  series  of  curves  and 
straight  lines. 

If  the  drum  is  rotated  with  the  lower  side  of  the  indica- 
tor piston  connected  to  atmosphere,  the  pencil  will  trace 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES    119 


a  horizontal  line.  This  is  known  as  the  atmospheric  line 
and  is  used  as  a  reference  for  locating  the  pressure  scale. 
If  the  indicator  cylinder  is  then  connected  with  the  engine 
cylinder  and  the  drum  is  rotated  by  the  reducing  mechanism, 
a  diagram  similar  to  that  of  Fig.  71  will  be  drawn  upon  the 
card.  The  atmospheric  line  indicates  the  height  assumed 
by  the  pencil  when  atmospheric  pressure  acts  on  the 
piston  and,  knowing  the  value  of  the  existing  atmospheric 
pressure  (barometer  reading)  and  the  scale  of  the  spring, 
a  line  at  a  height  representing  zero  pressure  can  be  drawn 
on  the  card.  This  line  is  indicated  in  Fig.  72. 

The  length  of  the  card  between  the  lines  a  and  b  is 
proportional  to  the  length  of  the  engine  stroke  and  there- 


£. 


Volume  Scale 

FIG.  72. 


fore  to  the  piston  displacement,  that  is,  to  the  volum,e 
swept  through  by  the  piston.  Knowing  the  clearance 
volume  of  the  engine  as  a  percentage  or  fraction  of  the 
piston  displacement,  this  fraction  of  the  length  of  the 
diagram  can  be  laid  off  from  the  end  of  the  diagram  to 
give  a  line  of  zero  volume.  This  line  is  also  indicated  in 
Fig.  72. 

With  the  line  of  zero  pressure  and  the  line  of  zero 
volume  drawn  in,  all  values  of  steam  pressure  and  volume 
occupied  by  steam  can  be  read  directly  from  the  diagram, 
and  it  thus  forms  a  picture  of  what  occurs  within  the  real 
engine  cylinder. 

The  indicator  diagram  is  used  for  a  number  of  purposes, 
the  more  important  being: 


120 


STEAM  POWER 


Length  of  Diagram — >j 


(1)  The   determination  of   the   energy  made   available 
within  the  cylinder,  that  is,  the  indicated  horse-power,  I.h.p. 

(2)  The  determination  of  the  amount  of  initial  conden- 
sation and  of  heat  interchanges  between  walls  and  cylinder. 

(3)  The  determination  of  what  is  known  as  the  diagram 
water  rate. 

(4)  The  study  of  the  operation  and  timing  of  valves. 
The  second  one  of  these  uses  has  already  been  considered 
in  Chapter  VII,  the  others  are  treated  in  succeeding  sections. 

64.  Determination  of  I.h.p.  The  lines  of  the  indicator 
diagram  show  by  their  height  the  pressures  or  forces  acting 
on  the  engine  piston  as  it  moves.  But  the  product  of  force 
by  distance  is  equal  to  work  and  these  lines  can  be  used  there- 
fore for  determining  the  net 
work  done  by  the  steam 
upon  the  piston. 

In  Fig.  73  is  shown  the 
upper  part  of  the  diagram, 
the  curved  lines  represent- 
ing the  successive  pressures 
in  pounds  per  square  inch 
which  acted  on  the  left  face 
of  the  piston  while  it  moved 
outward.  If  the  average 
pressure  could  be  deter- 
mined and  multiplied  by 
the  area  of  the  piston  face,  this  product  would  be  the 
average  total  force  acting  on  the  piston.  Multiplying  this 
by  the  distance  traveled  would  give  the  work  done  by 
the  steam  upon  the  piston.  Expressed  in  the  form  of  an 
equation, 

in  which 

#o  =  work  done  upon  piston  by  steam  during  outstroke; 
po  =  mean  pressure  (in  pounds  square  inch)   acting  on 
piston  during  outstroke; 


FIG.  73. — Positive  Work  Area. 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES    121 

a  =  area  of  piston  face  in  square  inches;   and 
L  =  stroke  of  piston  in  feet. 

For  the  instroke  shown  in  Fig.  74,  the  work  done  by 
the  piston  on  the  steam  is  given  by  the  similar  expression 


(34) 


FIG.  74. — Negative  Work  Area. 

in  which  E<  and  p«  represent  work  done  and  mean  pressure 
respectively. 

The  net  work  done  by  the  steam  upon  the  piston  per 
cycle  is  then, 


—  Et  =  (p0  —  pt)aL  ft.-lbs.    . 


(35) 


The  values  of  po  and  pt  can  be  found  directly  from  the 
diagram  by  dividing  the  areas  AQ  and  At  respectively  by  the 
length  I  and  then  multiplying  by  the  scale  of  the  spring, 
giving 

po  =  -r-X scale  of  spring, 
and 

Pi  =  -r~x scale  of  spring, 


122  STEAM  POWEE 

so  that, 

po  —  pt  =  p  =  —  —j  —  -  X  scale  of  spring        .     .     .     (36) 

area  of  diagram  vy  .       .  /0_, 

—  —  X  scale  of  spring.  .     (37) 

The  value  of  p  evidently  can  be  determined  very  simply 
from  the  indicator  diagram,  and  the  work  per  cycle  can  be 
found  when  p  is  known  by  substituting  in  the  following 
equation,  obtained  by  putting  p  for  po  —  pt  in  Eq.  (35), 

#cyoie  =  pXaXLft.-lbs  .....     (38) 

The  pressure  p  is  known  as  the  mean  effective  pressure  and 
is  often  represented  by  M.E.P. 

If  n  cycles  are  produced  per  minute,  the  net  work  done 
by  the  steam  upon  the  piston  per  minute  will  be 


,     .....     (39) 
which  is  generally  rearranged  to  read, 

Emm^pLan,      .......     (40) 

in  which  form  the  group  of  letters  forming  the  right-hand 
member  is  easily  remembered. 

Since  33,000  foot-pounds  per  minute  are  equivalent 
to  one  horse-power,  it  follows  that  the  power  made  avail- 
able as  shown  by  the  indicator  diagram,  that  i«,  the  indicated 
horse-power,  must  be, 


in  which 

p  =  mean  effective  pressure  in  pounds  per  square  inch  ; 

L  =  stroke  of  piston  in  feet; 

a  =  area  of  piston  in  square  inches; 

=  (diam.  cyl.  in  inches)2  XTT/  4  =  .7854d2;  and 
n  =  number  of  cycles  per  minute. 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES    123 

If  an  engine  cylinder  takes  steam  on  one  side  of  the 
piston  only,  that  is,  if  the  cylinder  is  single  acting,  the  num- 
ber of  cycles  produced  per  minute  is  equal  to  the  number 
of  revolutions  per  minute,  but  it  should  be  noted  that  for 
other  arrangements  this  is  not  necessarily  true.  In  the 
case  of  double-acting  engines  which  receive  steam  at  both 
ends  of  the  cylinder,  the  number  of  cycles  produced  is  equal 
to  twice  the  number  of  revolutions. 

It  should  also  be  noted  that  the  symbol  a  represents 
the  area  of  the  piston  face  upon  which  the  steam  acts. 
If  a  piston  rod  extend  from  the  face  of  the  piston  to  and 
through  the  cylinder  head  (as  is  always  the  case  at  the 
crank  end  of  double-acting  cylinders),  the  area  of  the 
piston  rod  must  be  subtracted  from  that  of  the  piston  to 
obtain  the  area  on  which  the  steam  really  acts.  When  a 
tail  rod  is  used,  a  correction  must  be  made  for  each  side 
of  the  piston. 

In  the  case  of  double-acting  engines  the  indicated 
horse-power  may  be  determined  in  two  ways:  It  may  be 
figured  separately  for  the  two  ends  of  the  cylinder,  or 
the  values  for  the  area  and  pressure  may  be  averaged  for 
the  two  ends  and  the  value  of  n  chosen  equal  to  twice 
the  revolutions  per  minute.  The  former  is  generally  the 
more  accurate  method. 

It  will  have  been  observed  that  the  area  of  the  indi- 
cator diagram  must  be  determined  before  the  mean 
effective  pressure  can  be  found.  This  area  is  generally 
measured  by  means  of  an  instrument  known  as  a  planimeter, 
and  this  is  the  most  accurate  method.  It  occasionally 
happens,  however,  that  a  planimeter  is  not  available  when 
the  value  of  the  indicated  horse-power  is  desired.  Under 
such  circumstances  an  approximate  determination  of  the 
area  of  the  indicator  diagram  can  be  made  by  the  method 
of  ordi nates. 

For  this  purpose  the  length  of  the  diagram  is  divided 
into  an  equal  number  of  parts,  usually  ten,  as  shown  in 


124 


STEAM  POWER 


Fig.  75  and  vertical  lines  3/1,  y2,  ys,  etc.,  are  drawn  at  the  center 
of  each  of  the  parts  into  which  the  diagram  has  been  divided. 
The  mean  ordinate  or  height  is  then  found  from  the  equa- 
tion, 


= 
m 


number  of  vertical  lines' 


(42) 


and  the  mean  effective   pressure  is  then   determined   by 
multiplying  ym  by  the  scale  of  the  spring. 

An  indicator  diagram  similar  to  that  shown  in  Fig.  76 
is  occasionally  obtained.  The  small  loop  on  the  end  repre- 
sents negative  work,  since  the  pressure  of  the  steam  which 


FIG.  75. 


FIG.  76. 


does  work  upon  the  piston  is  lower  than  that  which  resists 
the  return  of  the  piston.  When  using  a  planimeter,  this  area 
is  automatically  subtracted  from  that  of  the  rest  of  the 
diagram,  but  care  should  be  taken  to  see  that  this  is  also 
done  when  the  method  of  ordinates  is  used. 

ILLUSTRATIVE    PROBLEM 

1.  Determine  the  I.h.p.  of  a  double-acting  steam  engine,  having 
a  cylinder  8  ins.  diameter,  length  of  stroke,  12  ins.,  running  at 
100  R.P.M.,  the  mean  effective  pressure  (M.E.P.)  on  the  piston 
being  45  Ibs.  Neglect  the  area  of  the  piston  rod. 


I.h.p.  = 


pLan      (pXa)  Ibs.X(Lre)  ft.  per  min. 
33,000"       33,000  ft.-lbs.  per  min7~ 
(45X8X8X.7854)  Ibs.  XjfX  100X2)  ft.  per  min. 
33,000 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES    125 

2260  lbs.X200  ft.  per  min. 
33,000 

=  14  nearly. 

2.  The  I.h.p.  of  a  double-acting  engine  is  14,  the  R.P.M.  =100; 
M.E.P.  =45  Ibs.;  length  of  stroke  =  12  ins.  Find  the  diameter 
of  the  cylinder,  neglecting  area  of  piston  rod. 

First  determine  the  area  of  the  piston  from  the  formula 

plan  33,000  I.h.p. 

' 


pXLXn 
33,000X24 


.  (approx.). 


65.  Conventional    Diagram    and    Card    Factors.     It  is 

often  necessary  to  approximate  the  mean  effective  pressure 
obtained  in  the  cylinder  of  an  engine  when  no  indicator 
diagrams  are  available.  The  most  common  case  is  when 
an  engine  is  being  designed  to  carry  a  certain  load  and  it 
is  desired  to  determine  the  necessary  cylinder  dimensions 
and  speed.  If  the  probable  mean  effective  pressure  can  be 
determined,  the  dimensions  and  speed  can  be  found  from  the 
equation, 

,.    ,          ,      vLan 
I.h.p.  per  cylinder  end  =  £ 

by  rewriting  it 

pLnX0.7854d2 
33,000      ~ 
from  which 

33,000  Lh.fr 
"  0.7854  pLn  ' 

Since  n  is  equal  to  revolutions  per  minute  for  one  cylinder 
end,  the  product  of  L  by  n  must  be  equal  to  half  the  piston 


126  STEAM  POWER 

speed  of  the  engine,  and  a  proper  value  of  this  product  can 
be  chosen  for  substitution  in  the  equation.  If  a  proper 
value  for  p  can  then  be  predicted  the  only  unknown  remain- 
ing will  be  the  diameter  d,  and  this  can  be  found  by  solving 
the  equation. 

The  prediction  of  the  mean  effective  pressure  is  made 
either  by  drawing  upon  recorded  experience  in  the  form 
of  values  obtained  in  similar  engines  previously  constructed 
or  by  means  of  what  is  known  as  a  conventional  indicator 
diagram. 

The  conventional  diagram  is  drawn  with  upper  and 
lower  pressures  equal  to  those  expected  in  the  case  of  the 
real  engine,  and  all  expansions  and  compressions  are  drawn 
as  rectangular  hyperbolas.  The  equation  of  the  rectangu- 
lar hyperbola  is 

PlVi-PnVn, (44) 

in  which  subscript  1  indicates  initial  conditions  and  sub- 
script n  represents  any  later  conditions  with  the  same  mate- 
rial in  the  cylinder.  This  law  is  assumed  because  it  is  the 
simplest  and,  as  a  rough  average,  gives  values  as  close  to 
those  actually  attained  as  do  any  of  the  more  complicated 
laws. 

The  diagram  may  be  drawn  as  nearly  as  possible  like 
the  one  which  the  engine  may  be  expected  to  give  or  it 
may  be  drawn  with  various  simplifications  which  remove 
it  more  and  more  from  the  approximation  to  an  actual 
indicator  diagram.  In  any  case,  the  mean  effective  pres- 
sure is  determined  from  this  diagram  and  this  value  is  then 
multiplied  by  a  corrective  factor,  the  value  of  which  has 
been  determined  by  experience.  This  corrective  factor  is 
called  the  diagram  factor  or  card  factor  and  it  is  realty 
the  ratio  of  the  area  of  the  diagram  the  engine  would 
really  give  to  the  area  of  the  conventional  diagram 
used. 

The  simplest  form  of  conventional  diagram  is  drawn 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES    127 


by  neglecting  the  clearance  volume  and  has  the  shape  shown 

in  Fig.  77.     The  upper  line  is  drawn  horizontal  at  a  height 

representing  the  highest  pressure  expected  and  of  such  a 

length  (compared  with  the  length  of  the  diagram)   as  will 

approximately  represent  the  fraction  of  the  stroke  at  which 

cut-off    is   to   occur    in    the    real 

engine.     The  expansion  curve  is 

then  drawn   in    as  a   rectangular 

hyperbola  and  extended  until  the 

end   of   the    diagram  is    reached. 

The   next  line  is  drawn   vertical 

and  the  lower  line  of  the  diagram 

is   drawn    horizontal  at  a  height 

representing  the  pressure  expected 

in  the  space  into  which  the  engine 

is  to  exhaust. 

This  simple  diagram  can  be 
divided  into  the  three  areas  shown 
and  the  value  of  the  work  represented  by  these  areas  can  be 
determined  from  the  equations  given  below,  the  first  and 
last  of  which  should  be  self-evident  from  what  has  preceded. 
The  equation  for  the  work  represented  by  area  A  2  can  be 
determined  very  easily  by  means  of  integral  calculus.  The 
equations  are, 

AI  represents  P\V\  ft.-lbs.; 


FIG.  77. — Conventional  In- 
dicator Diagram. 


represents  PiFi  loge-^  =  PiFi  loger  ft.-lbs., 


and 


represents 


ft.-lbs., 


in  which  P  represents  pressure  in  pounds  per  square  foot  and 
V  represents  volume  in  cubic  feet. 

The  total  area  is  then  equal  to  the  sum  ^1+^2  —  ^3 
and  the  net  work  is  equal  to  a  similar  sum  of  the  right- 


128  STEAM  POWEE 

hand  members  given  above.  The  net  work  must  also 
equal  the  mean-effective  pressure  Pm  multiplied  by  the 
total  volume  change,  so  that 


\oger-P2V2,     .     .     (45) 
and 


-P2      ...     (46) 

P2,     .     .     .     (47) 
/ 

1         Vi 

and  substituting  —  for  -==-  this  gives 

-P2.  (48) 


V2 
The  ratio  y"  =  r  is  called  the  ratio  of  expansion  and  its 

FI     1 
reciprocal.  -=^-  =  —  is  known  as  the  cut-off  ratio.     By  the  use 

V  2       T 

of  this  ratio  the  volume  terms  can  be  disposed  of  and  the 
equation  above  is  obtained.  This  equation  then  gives  the 
mean  effective  pressure  in  terms  of  upper  and  lower  pres- 
sures and  the  fraction  of  the  stroke  at  which  cut-off  is 
desired  in  the  real  engine  and  no  cylinder  dimensions  need 
be  known. 

Since  pressures  in  steam-engine  practice  are  usually 
given  in  pounds  per  square  inch,  the  equation  for  mean 
effective  pressure  is  more  useful  in  the  form 


(49) 


in  which  p\  and  p2  and  pm  are  expressed  in  pounds  per 
square  inch  absolute.     For  convenience  in  the  use  of  this 


INDICATOE  DIAGRAM  AND  DERIVED  VALUES    129 


equation   the  values   assumed   by  the   bracketed  quantity 
are  given  for  various  conditions  in  Table  III. 


TABLE  III 


r 

1  +\oge  r 

r 

1  +loge  r 

r 

1  +loge  r 

r 

r 

r 

1.0 

1.00 

6.0 

0.465 

16.0 

0.236 

1.5 

0.937 

7.0 

0.421 

17.0 

0.226 

2.0 

0.847 

8.0 

0.385 

18.0 

0.216 

2.5 

0.766 

9.0 

0.355 

19.0 

0.208 

3.0 

0.700 

10.0 

0.330 

20.0 

0.200 

3.5 

0.644 

11.0 

0.309 

21.0 

0.192 

4.0 

0.597 

12.0 

0.290 

22.0 

0.186 

4.5 

0.556 

13.0 

0.274 

23.0 

0.180 

5.0 

0.522 

14.0 

0.260 

24.0 

0.174 

5.5 

0.492 

15.0 

0.247 

25.0 

0.169 

The  values  of  the  mean  effective  pressures  obtained 
from  this  form  of  diagram  are  very  much  higher  than  are 
to  be  expected  from  real  engines  with  the  same  initial  and 
terminal  pressures  and  the  same  nominal  ratio  of  expan- 
sion. They  are  therefore  corrected  by  multiplying  by  the 
proper  diagram  factor  as  selected  from  Table  IV.  It  is 
obvious  from  the  range  of  values  given  that  the  selection 
of  a  proper  value  for  the  factor  depends  largely  on  expe- 
rience, but  such  experience  is  quickly  gained  by  contact 
with  real  engines  and  a  study  of  the  practical  diagrams. 

TABLE  IV 
DIAGRAM  FACTORS 

Simple  slide-valve  engine 55  to  90% 

Simple  Corliss  engine 85  to  90 

Compound  slide-valve  engine 55  to  80 

Compound  Corliss  engine 75  to  85 

Triple-expansion  engine 55  to  70 

66.  Ratio  of  Expansion. — The  ratio  of  expansion  used 
above  is  sometimes  called  the  apparent  ratio.  It  is  not  the 


130 


STEAM  POWER 


real  ratio  of  expansion  for  an  engine  with   clearance. 
such  an  engine  the  real  ratio  of  expansion  is 


For 


FIG.  78. 


in  which  the  symbols  represent 
the  volumes  indicated  in  Fig. 
78. 

The  numerical  values  of  r 
and  r'  are  often  very  different 
and  care  should  be  used  in  dis- 
tinguishing between  them.  The 
diagram  factors  referred  to  in 
Table  IV  are  for  idealized  con- 


ventional cards  without  clearance  as  shown  in  Fig.  77. 


ILLUSTRATIVE   PROBLEMS 

1.  Given  an  engine  with  a  stroke  of  24  ins.  and  cut-off  occurring 
at  |  stroke.  Steam  pressure  of  160  Ibs.  per  square  inch  and 
back  pressure  of  16  Ibs.  Assume  diagram  factor  =80%.  Neglect- 
ing clearance,  find  the  probable  M.E.P. 


= 


160X.7-16  =  112.0-16=96lbs. 


Hence  probable  M.E.P.  =  .80X96  =76.8  Ibs. 

2.  A  given  double-acting  engine  indicates  75  I.h.p.  under  the 
following  conditions: 

Cut-off  at  20%;  steam  pressure,  140  Ibs.  per  square  inch 
absolute;  piston  speed,  600  ft.  per  minute;  back  pressure,  2  Ibs. 
per  square  inch  absolute. 

Assume  a  diagram  factor  for  this  type  of  engine  equal  to  85% ; 
and  neglecting  clearance,  find  a  convenient  size  of  the  cylinder 
(diameter  and  stroke). 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES    131 

Solution. 


_     /1+loge 


_^  _ 

I 


\        r       I  \       5 

=  73.1  -2  =71.1  Ibs.  per  sq.in. 

Diagram  factor  =85%.     Hence  probable 
M.E.P.=71.lX.85=60.41bs. 

Therefore,  since 

75X33,000 


d=9|  ins.  (approx.); 

and  since          2Ln  =  600,  assume  L  =  1  ft. 
hence  n=300R.P.M. 

The  engine  is  rated  9.5X12  ins.,  running  at  300  R.P.M. 

67.  Determination  of  Clearance  Volume  from  Diagram. 

It  was  shown  in  a  preceding  paragraph  that  the  clearance 
volume  of  a  cylinder  must  be  known  in  order  to  draw  the 
line  of  zero  volumes  on  the  indicator  diagram.  This 
volume  can  be  determined  accurately  for  any  real  engine 
by  weighing  the  quantity  of  water  required  to  fill  the  clear- 
ance space,  but  this  procedure  is  often  impossible  and 
an  alternative,  though  approximate,  method  is  often 
resorted  to. 

This  method  is  graphical  and  depends  upon  the  assump- 
tion of  the  law  of  expansion  and  compression.  As  in  the 
case  of  the  conventional  diagrams,  expansion  and  compres- 
sion are  assumed  to  follow  rectangular  hyperbolas. 

It  is  a  property  of  this  curve  that  diagonals  such  as 
aa  and  bb  drawn  for  rectangles  with  their  corners  on  the 


132 


STEAM  POWER 


curve  all  pass  through  the  origin  of  coordinates  as  shown 
in  Fig.  79. 

If  two  points  a  and  c  are  selected  on  the  expansion 
curve  of  a  real  diagram  and  a  rectangle  is  drawn  upon 
them  as  shown  in  Fig.  80,  the  diagonal  bd  extended  will 
pass  through  the  origin  of  coordinates,  if  the  expansion 
follows  the  assumed  law.  The  point  at  which  this  diagonal 
cuts  the  zero  pressure  line  must  therefore  be  the  point 
through  which  the  vertical  line  of  zero  volume  is  to  be  drawn. 

If  the  original  assumption  were  correct,  this  construc- 
tion would  give  the  same  point  when  different  locations 
of  the  points  a  and  c  were  chosen  and  when  used  on  the 


a  0 

FIG.  79. — -Rectangular  Hyperbola. 


FIG.  80. 


compression  as  well  as  on  the  expansion  line.  In  reality 
it  will  generally  give  as  many  different  locations  for  the 
origin  as  are  chosen  for  the  rectangle  abed.  It  is  customary 
to  construct  this  rectangle  of  fair  size  and  to  locate  it  near 
the  center  of  the  expansion  curve. 

68.  Diagram  Water  Rate.  As  was  shown  in  an  earlier 
chapter,  part  of  the  steam  supplied  an  engine  is  generally 
condensed  upon  the  cold  metal  walls  surrounding  it  The 
indicator  diagram  therefore  shows  the  volumes  assumed  by 
the  mixture  of  steam  and  liquid  water  in  the  cylinder, 
but,  since  the  volume  occupied  by  the  liquid  is  negligible, 
it  may  be  assumed  to  show  the  volumes  occupied  by  the 
part  of  the  mixture  which  exists  in  vaporous  form. 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES     133 

Assuming  that  the  vapor  is  saturated,  the  volume 
occupied  by  one  pound  at  various  pressures  can  be  found 
from  the  steam  tables  and,  therefore,  the  weight  existing 
in  the  cylinder  can  be  calculated.  The  weight  of  steam 
determined  in  this  way  is  known  as  the  indicated  steam, 
the  diagram  steam  or  the  diagram  water  rate. 

The  diagram  water  rate  is  generally  determined  for  a 
point  such  as  z  in  Fig.  81  just 
after  cut-off,  though  some 
engineers  prefer  to  use  a  point 
nearer  the  lower  end  of  the 
expansion  curve.  The  volume 
occupied  by  the  steam  con- 
tained in  the  cylinder  at  point 
z  is  equal  to  Vz  and  its  weight 
can  be  determined  by  dividing 
this  volume  by  the  specific  volume  V2  for  the  existing 
pressure  P2.  Thus,  calling  the  weight  of  eteam  in  the  cyl- 
inder Wz,  / 

«*-£.    V     ,,,v  .. '..    .     .     (51) 


FIG.  81. 


This  quantity  of  steam  is  a  mixture  of  cylinder  feed 
and  clearance  or  cushion  steam  and  the  weight  of  the  latter 
must  therefore  be  subtracted  from  wz  to  obtain  the  weight 
of  cylinder  feed  wf.  Assuming  the  cushion  steam  dry  and 
saturated  at  the  point  k,  the  weight  of  cushion  steam  is 


(52) 


so  that  the  weight  of  cylinder  feed  per  cycle  as  shown  by 
the  diagram  at  the  point  z  is 

V       T7 

_     _     _.JLf__!_*  /KO\ 


The  formula  is  generally  modified  to  give  the  steam 
consumption   per  indicated   horse-power   hour,   instead   of 


134  STEAM  POWER 

per  cycle,  and  it  is  also  expressed  in  different  terms  as  a 
matter  of  convenience. 
For  this  purpose  let 

ija  =  clearance  volume  divided  by  piston  displacement  per 
stroke 


?/2  =  piston  displacement  to  point  z  divided  by  piston  dis- 

placement per  stroke 
=  h 
I,' 
yk  =  piston  displacement  to  point  k  divided  by  piston  dis- 

placement per  stroke 
_l* 
I,' 

a  =  area  of  piston  in  square  inches  ; 
p  =  mean  effective  pressure  in  pounds  per  square  inch; 
L  =  stroke  in  feet  ;   and 
n  =  number  of  cycles  per  minute. 

The  piston  displacement  is  then  TTT^  cubic  feet   and 
the  volumes  at  z  and  k  are  given  by 


and 

aL 


Substituting  these  values  in  Eq.  (53)  gives  the  cylinder 
feed  per  cycle  as 

aL(y+ya    y*+yc\  , 

Wf  m\  v,     v,  )- 

Multiplying   by   the   number   of   cycles   per   hour   (60 Xn) 
and  dividing  by  the  indicated  horse-power,   ~-  jr— ,  gives 

oo,UUU 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES     135 


the  diagram  water  rate,  or  steam  shown  by  the  diagram  per 
I.h.p.  hour  as 


=  13,750  (ijz+yci    yi+ya\ 


,     .     .     .     (55) 


in  which  form  the  equation  involves  only  values  which  can 
be  determined  directly  from  the  diagram  without  any 
knowledge  of  the  engine  dimensions. 

The  value  obtained  for  wd  will  vary  as  the  location  of 
points  z  and  k  are  varied  because  of  the  quality  changes 
occurring  during  expansion  and  compression,  and  it  is 
obvious  that  the  diagram  water  rate  is  in  no  sense  an  accu- 
rate measure  of  the  real  water  rate  of  the  engine.  It  is, 
however,  very  useful  for  comparison  with  the  real  water 
rate,  the  ratio  giving  an  indication  of  the  loss  by  con- 
densation. 

Average  values  for  real  water  rates  are  given  in  Chapter 
XL 


ILLUSTRATIVE   PROBLEM 

Given  the  diagram  shown  in  Fig.  82  and  the  following  data 
from  an  actual  test,  find  the  diagram  water  rate  for  point  c,  and 
for  point  n.     Double-acting  steam 
engine  having: 

Average  piston  area  =  28.9  sq.- 
in.; 

Length  of  stroke  =8  in. ; 

R.P.M.  =237;         I.h.p.  =8.75; 
M.E.P.  =31.6  Ibs.; 

Clearance  =13%;  Beginning  of 
compression  =  29% ; 

Weight  of  condensate  per  hour 
=371  Ibs.; 

Quality  at  throttle  =95%; 

Sp.  vol.  at  c=7.8; 

Sp.  vol.  at  n  =12.57; 

Sp.  vol.  at  K  =38.4  cu.ft.  per  lb.; 

Assume  %  =  100%. 


FIG.  82. 


136  STEAM  POWER 

Solution.     Substitution  in  Eq.  (55)  gives 

,      ,    _13,750/?/c+?/cz     2/*+2/cz 

_13,750/0.39+0.13_0.29+0.13 
=  31.6  \       7.8  38.4 

=24.2  Ibs.  per  I.h.p.  per  hour  at  point  c. 


Q1    _  T/ 

01. D     \       Kn  1/fc 

13,750/0.638+0.13     0.29+0.13 


31.6   \      12.57  38.4 

=  21.83.  Ibs.  per  I.h.p.  per  hour  at  pcint  n. 

371 

Real  water  rate  = X0.95  =40.2  Ibs. 

&75 

69.  TV-diagram  for  a  Real  Engine.  In  Chapter  VI 
the  T^-diagrams  of  the  various  ideal  cycles  were  given  and 
attention  was  called  to  the  fact  that  these  diagrams  were 
.  particularly  useful,  because  they  showed  certain  things 
which  were  not  apparent  from  the  more  common  PV- 
diagrams. 

It  has  been  customary  for  many  years  to  draw  TV- 
diagrams  for  real  engines  by  "  transferring  "  the  PV- 
diagram  to  TV-coordinates,  and  various  analytical  and 
graphical  methods  have  been  developed  for  this  purpose. 
There  are  certain  unavoidable  errors  in  all  the  methods 
used  for  drawing  these  diagrams,  and  the  expansion  curve 
is  the  only  one  of  all  the  lines  finally  obtained  which  has 
any  claim  to  accuracy.  Even  this  curve  is  generally  incor- 
rectly interpreted,  because  a  knowledge  of  the  exact  weight 
of  clearance  steam  is  necessary  for  an  accurate  interpreta- 
tion and  such  knowledge  is  never  available. 

Under  the  circumstances  it  seems  unnecessary  to  con- 
sider in  this  book  the  rather  complicated  details  involved 
in  the  construction  of  T ^-diagrams  purporting  to  show 
the  behavior  of  steam  in  real  engines. 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES    137 

70.  Mechanical  and  Thermal  Efficiencies.  The  method 
of  obtaining  the  indicated  horse-power  from  the  indicator 
diagram  has  been  given  in  preceding  paragraphs.  In  the 
real  engine  this  power  is  not  all  made  available  at  the  shaft, 
because  some  of  It  is  used  in  driving  the  engine  against 
its  own  frictional  resistance.  Calling  the  power  lost  in 
this  way  the  friction  horse-power,  it  follows  that 

I.h.p.  =  F.h.p.+D.h.p,      ....     (56) 

in  which 

I.  h.  p.  =  indicated  horse-power  determined  from  the  real 

indicator  diagram; 
F.h.  p.  =  friction    horse-power,    i.e.,    power    required    to 

drive  engine;    and 

D.h.p.  =  developed  horse-power,  i.e.,  power  made  avail- 
able at  shaft. 

The  developed  horse-power  is  therefore  always  less 
than  the  indicated  horse-power.  The  better  the  construc- 
tion of  the  engine  the  smaller  the  friction  loss,  and  the 
measure  of  this  loss  is  usually  given  in  the  form  of  an  ef- 
ficiency. It  is  called  the  mechanical  efficiency,  and  is  defined 
by  the  equation 


(57) 


Values  of  mechanical  efficiency  range  from  about  80  per 
cent  in  the  case  of  poorly  designed  and  poorly  adjusted 
horizontal  engines  to  about  95  per  cent  in  the  case  of  the 
best  vertical  designs. 

The  efficiency  determined  by  dividing  energy  made 
available  by  heat  supplied  is  known  as  the  thermal  efficiency. 
There  are  two  possible  thermal  efficiencies,  one  based  on 
the  indicated  power  and  the  other  on  the  developed  power. 
The  former  is  called  the  thermal  efficiency  on  the  indicated 
horse-power  or  the  indicated  thermal  efficiency;  the  other 


138  STEAM  POWER 

is  known  as  the  thermal  efficiency  on  the  developed  horse- 
power  or  the  developed  thermal  efficiency.     Obviously 

Dev.  ther.  eff.  =  Mech.  eff.X  Indie,  ther.  eff.    .     .     (58) 

The  heat  supplied  may  be  assumed  in  two  different 
ways;  it  may  be  taken  as  the  total  heat  above  32°  F.  in  the 
steam  supplied  the  engine,  or  it  may  be  taken  as  this  value 
less  the  heat  of  the  liquid  corresponding  to  exhaust  tem- 
perature. The  second  method  is  preferable,  since  it  is 
reasonable  to  assume  that  the  exhaust  steam  can  be  con- 
densed to  water  at  the  same  temperature  and  that  this  water 
can  be  pumped  to  the  boiler  with  the  heat  of  the  liquid 
corresponding  to  this  temperature.  This  is  practically 
parallel  to  the  assumption  made  in  treating  the  theoretical 
cycles. 

The  thermal  efficiencies  are  then 


Indie,  ther.  eff. 

I.h.p.X2545 


Heat  above  exhaust  temp,  supplied  per  hour' 
and 

Dev.  ther.  eff. 

= D.h.p.X2545 

Heat  above  exhaust  temp,  supplied  per  hour* 


(59) 


(60) 


Values  of  the  indicated  thermal  efficiency  range  from 
about  5  per  cent  in  ordinary  practice  with  small  engines  to 
about  25  per  cent  in  the  best  large  engines.  Values  as  low 
as  1  per  cent  are  not  uncommon  with  small,  poorly  designed 
and  poorly  operated  engines. 

The  actual  performance  of  the  cylinder  of  an  engine 
is  sometimes  compared  with  the  ideal  possibilities  as  indi- 
cated by  the  Clausius  and  the  Rankine  cycles.  The  ratio 
of  the  work  obtained  in  the  real  engine  to  that  which  could 
be  obtained  from  the  same  quantity  of  heat  with  a  Rankine 


INDICATOR  DIAGRAM  AND  DERIVED  VALUES     139 

or  Clausius  cycle  is  a  measure  of  the  performance  of  the 
real  cylinder.  This  ratio  is  variously  designated  as  cylinder 
efficiency,  indicated  efficiency,  relative  efficiency,  etc.  Its 
values  range  from  less  than  40  per  cent  to  over  80  per  cent, 
the  highest  recorded  value  being  just  over  88  per  cent. 

PROBLEMS 

1.  Using  Table  I,  Chapter  I,  plot  the  specific  heat  of  water 
between  the  range  of  temperatures  of  20°  F.  and  300°  F.  for  the 
intermediate  values  given.     By  the  ordinate  method  for  finding 
the  mean  height  of  an  indicator  diagram,  determine  the  mean 
or  average  specific  heat  over  this  range. 

2.  A  double-acting  engine  is  required  to  give  50  I.h.p.  under 
the  following  conditions: 

Cut-off  =25%; 

Steam  pressure  =  150  Ibs.  per  square  inch  absolute; 
Back  pressure  =  16  Ibs.  per  square  inch  absolute; 
Piston  speed  =540  ft.  per  minute. 

If  the  diagram  factor  for  this  type  of  engine  is  75%,  find  the 
diameter  of  the  cylinder  and  select  the  stroke  and  R.P.M. 

3.  Assume  a  single-acting  engine  with  10-in.  diameter  and  12-in. 
stroke,  10X12  ins.,  to  have  cut-off  occur  at  various  points  between 
10%  and  50%  of  stroke.     Assume  also  the  pressures,  speed,  and 
card  factor  as  given  in  Prob.   2.     Find  the  probable  I.h.p.  at 
different  cut-offs. 

4.  Given  an  18X24-in.  engine  running  at  120  R.P.M. 
Back  pressure  =2  Ibs.  per  square  inch  absolute; 
Clearance  =  10%; 

Cut-off  =40%; 

Diagram  factor  =85%. 

Supposing  cut-off  to  remain  constant,  find  the  I.h.p.'s  cor- 
responding to  steam  pressure  of  50,  90,  and  130  Ibs.  per  square 
inch  absolute. 

5.  Find  the  weight  of  dry  steam  which  must  be  supplied  per 
I.h.p.  hour  for  each  case  of  the  previous  problem,  assuming  the 
quality  at  cut-off  to  be  80%.     Assume  compression  pressure  to 
be  30  Ibs.  absolute  and  that  steam  is  dry  and  saturated  at  end  of 
compression. 

6.  Find  the  quality  of  steam  at  cut-off  in  a  cylinder,  in  which 
the  piston  displacement  is  0.1278  cu.ft.;   clearance  =  10%;   cut-off 
at  25%  stroke;    steam  pressure  at  cut-off,  115  Ibs.  per  square 


140  STEAM  POWER 

inch  absolute,  and  weight  of  steam  in  the  cylinder  at  cut-off  = 
0.012  Ib. 

Actual  vol. 

JNote.     Quality  =—  — —  for  the  given  pressure. 

W  eight  XSp.  vol. 

7.  The  piston  displacement  of  a  certain  engine  is  0.2  cu.ft. 
What  weight  of  steam  is  in  the  cylinder  at  release  where  quality 
is  90%,  and  pressure  is  25  Ibs.  per  square  inch  absolute,  if  the 
clearance  is  10%,  and  release  occurs  at  95%  of  the  stroke? 

8.  Find  the  weight  of  cushion  steam  in  a  6X6  in.  engine  in 
which  clearance  =  15%;    compression  begins  at  85%  of  the  return 
stroke;    back  pressure  is  14.7  Ibs.  per  square  inch  absolute,  and 
the  quality  of  the  cushion  steam  at  the  beginning  of  compression 
is  95%. 

9.  Find  the  pressure  and  quality  at  the  end  of  the  compression 
line  of  the  previous  problem,  assuming  it  to  be  adiabatic. 

10.  An  8X 10  in.  engine  running  at  300  R.P.M.  is  double-acting, 
and  cuts  off  at  15%  of  the  stroke  at  a  pressure  of  120  Ibs.  per 
square  inch  absolute.     It  has  a  steam  consumption  of  35  Ibs.  per 
I. h. p. -hour.     The  compression  begins  at  60%  of  the  return  stroke 
with  a  quality  of  unity  and  a  back  pressure  of  5  Ibs.  per  square 
inch  absolute.     Clearance  =  10%. 

If  this  engine  delivers  27  H.P.,  and  has  a  mechanical  efficiency 
of  90%,  what  is  the  quality  at  the  point  of  cut-off? 

11.  In  the  previous  problem,  assume  release  to  occur  at  90% 
of  the  stroke  with  an  absolute  pressure  of  30  Ibs.  per  square  inch. 
What  is  the  quality  at  this  point? 

12.  A  certain  engine  gives  one  horse-power  hour  at  the  shaft 
for  every  20  Ibs.  of  steam  supplied.     The  steam  has  an  initial 
pressure  of  150  Ibs.  absolute  and  is  dry  and  saturated  when  it 
arrives  at  the  engine.    The  back  pressure  against  which  steam  is 
exhausted  is  4  Ibs.  absolute. 

(a)  Find  the  thermal  efficiency  of  this  engine  on  the  developed 
or  shaft  horse-power. 

(6)  If  the  mechanical  efficiency  of  the  engine  is  90%,  what 
is  the  value  of  the  thermal  efficiency  on  the  indicated  horse-power? 


CHAPTER  IX 


COMPOUNDING 

71.  Gain  by  Expansion.  The  cycle  which  gives  a 
rectangular  PF-diagram  is  the  least  economical  of  all  the 
ideal  cycles  described  in  Chapter  IV.  This  comes  from 
the  fact  that  none  of  the  heat  stored  in  the  steam  is  con- 
verted into  work  when  this  cycle  is  used.  Thus,  if  the 
cylinder  shown  by  full  lines  in 
Fig.  83  operate  on  this  cycle 
and  be  of  such  size  that  it  will 
receive  just  one  pound  of  steam 
per  cycle,  it  makes  available  an 
amount  of  work  represented  by 
the  area  abed.  The  positive 
work  done  by  the  steam  upon 
the  piston  is  the  equivalent  of 
the  external  latent  heat  of 
vaporization  while  no  use  is 
made  of  the  heat  stored  in  the 
steam.  This  stored  heat  is  re- 
moved as  heat  during  the  con- 
densation and  exhaust,  which  give  the  lines  be  and  cd. 

If  a  piece  be  added  to  the  cylinder  as  indicated  by 
the  dotted  lines,  the  same  quantity  of  steam  will  make  more 
heat  available  by  expanding  after  cut-off,  as  shown  by  the 
curve  be,  the  net  work  in  this  case  being  represented  by 
the  area  abefd  instead  of  by  the  smaller  area  abed.  But 
the  heat  supplied  is  the  same  in  both  cases,  namely  that 
required  to  form  one  pound  of  steam  at  the  pressure  PI, 
so  that  the  use  of  a  large  cylinder  and  the  incomplete  ex- 

141 


FIG.  83. 


142 


STEAM  POWER 


pansion  cycle  results  in  the  development  of  more  work 
than  can  be  obtained  with  the  rectangular  cycle  from  the 
same  amount  of  heat. 

Obviously  it  would  be  theoretically  advantageous  to 
add  still  more  to  the  length  of  the  cylinder  and  allow  the 
expansion  to  continue  to  back  pressure,  giving  the  com- 
plete expansion  cycle  as  shown  in  Fig.  84,  thus  obtaining 
the  maximum  quantity  of  work  at  the  expense  of  the  heat 
stored  in  the  steam  supplied  the  cylinder.  Practically, 
it  is  found  inadvisable  to  continue  the  expansion  to  such  a  de- 


FIG.  84. 

gree  in  reciprocating  steam  engines,  because  at  low  pressures 
the  volume  increases  very  rapidly  for  small  pressure  drops. 
Thus  a  great  increase  is  necessary  in  the  size  of  the  cylinder 
if  the  last  part  of  the  expansion  is  to  be  completed,  but 
the  amount  of  work  obtained  is  comparatively  small,  as 
shown  by  the  small  height  of  the  long  toe  thus  added  to 
the  diagram.  This  may  result  in  an  actual  loss,  because  the 
increased  friction  losses  of  the  very  large  cylinder  may 
more  than  balance  the  small  increase  of  net  work  gained 
by  its  use.  It  thus  results  that,  in  every  real  reciprocating 
engine,  there  is  some  point  beyond  which  it  is  not  economi- 
cal to  carry  the  expansion,  and  the  incomplete  expansion 


COMPOUNDING 


143 


FIG.  85. 


cycle  is  therefore  approximated  in  such  engines  rather 
than  the  cycle  with  complete  expansion. 

Viewing  the  matter  from  another  angle,  a  cylinder  of 
a  certain  size  may  be  assumed  as  shown  in  Fig.  8-",.  The 
use  of  the  rectangular  cycle 
abed  in  this  cylinder  will  make 
available  the  maximum  quan- 
tity of  work  possible  with  the 
upper  and  lower  pressures 
chosen.  If  cut-off  be  made 
to  occur  earlier  as  at  &',  the 
expansion  b'c'  will  result  in  a 
loss  of  the  quantity  of  work 
obtained,  as  shown  by  the 
area  b'bc',  but  the  steam  used 
per  horse-power  will  be  less, 

so  that  there  will  be  a  gain  in  steam  economy.  Putting  the 
cut-off  still  earlier  will  cause  a  still  greater  loss  of  work 
obtained  from  a  cylinder  of  the  chosen  size,  but  theoreti- 
cally will  result  in  greater  economy  of  steam. 

Summing  up,  it  may  be  said  that  the  greater  the  ratio 
of  expansion  the  greater  should  be  the  economy  in  the  use 
of  steam  on  a  theoretical  basis. 

The  lower  pressure  is  set  in  real  engines  by  the  pressure 
in  the  space  into  which  the  engine  is  to  exhaust.  If  the 
engine  is  to  be  operated  non-condensing,  the  atmospheric 
pressure  determines  the  lowest  possible  exhaust  pressure; 
if  the  engine  is  to  be  operated  condensing,  the  exhaust 
pressure  is  set  by  the  lowest  pressure  which  can  be  eco- 
nomically maintained  in  the  condenser. 

There  is  thus  a  real  limit  to  the  extent  to  which  expan- 
sion can  be  carried  in  any  real  engine  with  a  given  initial 
pressure.  A  certain  drop  must  exist  at  the  end  of  the 
diagram,  for  reasons  already  explained,  and  an  expan- 
sion line  drawn  backward  from  the  top  of  the  line  repre- 
senting this  drop  will  give  the  earliest  possible  cut-off 


144  STEAM  POWEE 

which   can  be    used    in   the    engine    with    a    given   initial 
pressure. 

The  ratio  of  expansion  can  be  further  increased,  how- 
ever,  by  raising  the  initial  pressure  as  indicated  by  the 

dotted    lines   in   Fig.   86, 
pi n  anci  the  limit  in  this  direc- 


tion would  come  with  the 
inability  of-  materials  of 
construction  to  withstand 
the  resulting  strains. 

These       conclusions 
drawn     from     the     facts 
V       developed  above  must  all 

-P,  be  modified  in  the  case  of 

*IG.  8b.  . 

real  engines,  because  of 

the  effect  of  cylinder  condensation.  This  has  been  shown 
to  increase  as  the  cut-off  is  made  earlier  and  as  the 
pressure  (and  therefore  the  temperature)  range  in  a  cyl- 
inder is  increased.  There  is,  therefore,  a  limit  beyond 
which  it  is  not  advisable  to  carry  the  ratio  of  expansion  in 
a  single  cylinder. 

Experience  has  shown  that  the  best  commercial  results 
are  obtained  from  simple  engines,  that  is,  those  expanding 
the  steam  entirely  in  one  cylinder,  when  (a)  they  are  operated 
non-condensing,  (b)  the  initial  pressure  is  between  80  and  100 
pounds  per  square  inch  for  the  simpler  forms  of  valves  and  up 
to  125  Ibs.  with  the  better  forms  of  valves,  and  (c)  the  point 
of  cut-off  is  at  about  J  stroke  with  the  simpler  valves  and  at 
from  I  to  \  stroke  with  the  better  forms  of  valves.  These 
values  of  cut-off  correspond  to  nominal  expansion  ratios 
of  5  and  4  respectively  and  to  lower  values  when  clearance 
is  taken  into  account. 

72.  Compounding.  If  the  ratio  of  expansion  is  to  be 
increased  above  the  values  just  given,  some  means  must 
be  used  for  the  reduction  of  loss  by  condensation.  This 
loss  can  be  reduced  by  decreasing  the  surface  exposed  to 


COMPOUNDING 


145 


high-temperature  steam  and  by  decreasing  the  temperature 
range  in  a  cylinder.  Both  of  these  results  can  be  achieved 
by  what  is  known  as  compounding. 

Assume  that  it  is  deemed  advisable  to  produce  a  cycle 
similar  to  that  shown  in  Fig.  87  (clearance  neglected)  and 
that  in  order  to  obtain 
high  steam  economy  (low 
water  rate)  the  ratio  of 
expansion  chosen  is  very 
much  greater  than  four. 
No  gain  in  economy 
would  result  from  such 
excessive  expansion  in  a 
single  cylinder,  in  fact 
there  would  be  a  well- 
defined,  unavoidable  loss. 
But  suppose  that  the  high- 
pressure  steam  is  admitted 
to  a  small  cylinder  such 
as  that  shown  and  is  ex- 
panded to  the  point  /,  is 


High  Pre&sure 

Cylinder 
H.P.  Cylinder 


Low  Pressure  Cylinder 
L.P.  Cylinder 


FIG.  87. 


then  exhausted  as  shown 
by  fg  into  the  larger  cyl- 
inder along  gf  and  then  expanded  to  the  point  c  in  the 
larger  cylinder.  The  cycle  produced  is  the  same  as  that 
which  would  have  been  obtained  by  expanding  entirely  in 
one  cylinder,  but  the  surface  of  the  clearance  space  of 
the  high-pressure  (H.P.)  cylinder,  which  is  exposed  to  high- 
pressure  steam  is  smaller  than  it  would  be  in  a  cylinder  of 
the  size  required  to  hold  the  steam  when  fully  expanded  and, 
moreover,  the  lowest  temperature  to  which  it  is  subjected  is 
that  corresponding  to  the  pressure  at  /  instead  of  the 
much  lower  temperature  corresponding  to  the  pressure 
at  d. 

The  condensation  which  would  occur  in  the  H.P.  cylinder 
would  obviously  be  less  than  that  which  would  result  from 


146  STEAM  POWER 

the  use  of  one  large  cylinder  and,  remembering  that  the 
greater  part  of  the  heat  given  up  during  condensation  is 
received  again  by  the  steam  during  exhaust,  it  is  obvious 
that  approximately  this  same  quantity  of  heat  can  again 
be  given  to  the  low-pressure  cylinder  walls.  Thus,  although 
there  are  two  cylinders  in  which  condensation  and  re- 
evaporation  occur,  and  although  the  sum  of  the  heat  given 
to  the  walls  of  the  high-pressure  cylinder  and  that  given 
to  the  walls  of  the  low-pressure  cylinder  might  be  greater 
than  that  given  to  the  walls  of  a  single  cylinder  under  similar 
conditions,  the  use  of  two  cylinders  results  in  a  consider- 
able saving  because  loss  in  the  high-pressure  cylinder  is 
practically  wiped  out  by  the  exhaust  of  the  heat  concerned 
into  the  low-pressure  cylinder. 

If  the  loss  by  radiation  and  conduction  from  the  high- 
pressure  cylinder  be  neglected,  the  result  of  the  use  of  two 
cylinders  is  practically  to  limit  the  loss  by  condensation 
and  re-evaporation  to  that  occurring  in  the  low-pressure 
cylinder.  As  the  ratio  of  expansion  in  this  cylinder  is  in 
the  neighborhood  of  that  common  in  simple  engines,  or 

even  less,  and  as  the  tem- 
perature range  is  small, 
the  net  loss  is  also  small. 
It  is  obvious  that  the 
smaller  the  surface  of  the 
high-pressure  cylinder  can 
be  made,  and  the  smaller 
the  temperature  range  in 
a  single  cylinder,  the 
smaller  will  be  the  net  loss 
FIG.  88.  by  cylinder  condensation 

and    re-evaporation.       A 

saving  should  therefore  be  effected  by  using  more  than  two 
cylinders,  and  it  is  not  inconceivable  that  five  or  more  might 
be  used.  The  result  of  using  five  cylinders  is  shown  in  -Fig. 
88,  and  it  is  evident  that  the  clearance  surfaces  exposed  to 


COMPOUNDING  147 

high  temperatures,  the  temperature  ranges  per  cylinder 
and  the  ratios  of  expansion  per  cylinder  are  all  small. 
The  gain  in  economy  should  therefore  be  correspondingly 
great. 

There  are  two  limits  to  the  possible  multiplication  of  cylin- 
ders in  this  way. 

(1)  As  the  number  increases  the  radiating  surface  and 
therefore  the  heat  lost  by  radiation  increases.     The  extent 
of  this   effect   can   be   appreciated   by  noting   that   every 
cylinder  with  the  exception  of  the  low-pressure  cylinder  is 
really  an  unnecessary  addition,  because  the  cycle  could  be 
produced   entirely  in   the   low-pressure   cylinder.     On  the 
other  hand,  the  surfaces  of  cylinders  which  operate  at  high 
temperature  are  small  as  compared  with  that  which  would 
be  exposed  to  this  temperature  if  the  entire  cycle  were  pro- 
duced in  the  low-pressure  cylinder. 

(2)  As  the  number  of  cylinders  is  increased,  the  first 
cost,  the  complexity  and  the  cost  of  lubrication  and  attend- 
ance are  all  increased  so  that,  for  each  installation,  some 
number  will  be  found  beyond  which  the  interest  on  the 
investment  and  the  added  cost  of  operation  and  mainte- 
nance would  more  than  balance  the  saving  of  fuel. 

The  second  limit  mentioned  is  the  more  important 
commercially,  as  it  is  the  first  one  reached.  For  ordinary 
operating  conditions  in  stationary  power  plants  expansion 
in  two  cylinders  generally  gives  the  most  economical  results. 
The  total  ratio  of  expansion  is  generally  between  7  and  16, 
that  is,  the  volume  of  steam  at  release  in  the  L.P.  cylinder 
is  from  7  to  16  times  the  volume  at  cut-off  in  the  H.P. 
cylinder.  For  large  pumping  stations  and  large  marine 
installations,  expansion  in  three  cylinders  is  generally 
considered  the  most  economical,  and  total  ratios  of  expansion 
of  20  or  more  are  used.  Four  and  five  cylinders  have 
been  used,  but  the  resultant  gains  do  not  seem  to  warrant 
any  extensive  installation  of  such  units. 

Engines  using  more  than  one  cylinder  for  the  expansion 


148  STEAM  POWEK 

of  steam  in  the  way  just  described  are  called  multi-expan- 
sion engines,  or  compound  engines,  and  the  use  of  multi- 
expansion  is  spoken  of  as  compounding.  Custom  has 
almost  confined  the  use  of  the  term  compound  engine 
to  those  in  which  only  two  cylinders  are  used  in  series  as 
indicated  in  Fig.  89,  and  such  engines  are  often  spoken  of 
as  2x  engines. 

Engines  in  which  three  cylinders  are  used  in  series 
are  called  triple-expansion  or  3x  engines.  With  four  and 
five  cylinders  in  series  the  engines  are  known  as  quadruple 
or  4z  and  quintuple  or  5x,  respectively. 

In  the  case  of  triple-expansion   engines  of  large  size, 


H.P.  Exhaust  and  L.P.  Admission 


To 
Condenser  • 

FIG.  89.  FIG.  90. 

the  volume  of  the  low-pressure  cylinder  required  generally 
becomes  so  great  that  it  is  found  economical  to  use  two 
low-pressure  cylinders  instead  of  one.  The  flow  of  steam 
in  such  an  engine  is  represented  diagrammatically  in  Fig. 
90.  This  type  is  known  as  a  four-cylinder,  triple-expansion 
engine. 

All  multi-expansion  engines  are  generally  operated 
condensing,  and  the  choice  of  type  is  determined  partly  by 
the  character  of  work  to  be  done  and  partly  by  economical 
considerations.  In  all  cases  the  boiler  pressure  must  be 
chosen  to  suit  the  type  of  engine  used.  The  pressures 
ordinarily  used  with  the  different  types  are  given  in 
Table  V. 


COMPOUNDING  149 

TABLE  V 
BOILER  PRESSURE  COMMONLY  USED 


Type  of  Engine. 

Boiler  Pressure. 
Pounds  per  Sq.in.  Gauge. 

Simple  

80  to  125 

High-speed  compound. 

100  to  170 

Low-speed  compound  
Triple  expansion  and  higher 

125  to  200 
125  to  225 

73.  The  Compound  Engine.  The  term  compound 
engine  will  be  used  hereafter  in  the  commercial  way  as 
referring  to  a  2x  engine.  Such  engines  may  roughly  be 
divided  into  two  types,  receiver  and  non-receiver  engines. 
The  latter  are  often  called  Woolf  engines,  after  the  man 
who  first  used  this  construction. 

A  receiver  engine  has  a  vessel  known  as  a  receiver 
located  between  the  two  cylinders  and  so  connected  with 
them  that  the  high-pressure  cylinder  exhausts  into  the 
receiver  and  the  low-pressure  cylinder  draws  its  steam 
from  the  receiver.  By  using  a  receiver  the  cylinders  are 
made  independent  of  each  other  so  far  as  steam  events 
are  concerned;  the  high-pressure  cylinder  can  exhaust 
at  any  time  with  reference  to  the  events  occurring  in  the 
low-pressure  cylinder. 

A  Woolf  type  has  practically  no  receiver,  the  high-pres- 
sure cylinder  exhausting  directly  into  the  low-pressure 
cylinder  through  the  shortest  convenient  connecting  pass- 
age. As  the  high-pressure  cylinder  must  exhaust  directly 
into  the  low-pressure  cylinder  it  follows  that  cut-off  must 
not  occur  in  the  latter  until  compression  starts  in  the 
former;  i.e.,  very  near  the  end  of  the  stroke. 

An  engine  with  a  receiver  of  infinite  size  would  give 
a  horizontal  exhaust  line  for  the  high-pressure  cylinder 
and  a  horizontal  admission  line  for  the  low-pressure  cylinder, 
since  the  small  amount  of  steam  given  to  or  taken  from  the 


150 


STEAM  POWER 


receiver  would  have  no  appreciable  effect  upon  the  pressure 
within  that  vessel.  Neglecting  throttling  losses,  the  high- 
pressure  and  low-pressure  cards  would  therefore  fit  together 
as  originally  indicated  in  Fig.  86. 

With  receivers  of  finite  size  there  are  pressure  changes 
during  exhaust  by  the  high-  and  admission  to  the  low-pres- 
sure cylinders,  and  real  valves  and  connections  also  cause 
certain  throttling  losses,  so  that  the  lines  representing 
these  events  are  not  horizontal  nor  do  they  exactly  coincide. 

A  diagrammatic  arrangement  of  the  Woolf  engine 
is  given  in  Fig.  91  with  idealized  diagrams  obtained  by 


FIG.  91. 

assuming  hyperbolic  expansions,  no  clearances,  and  no 
throttling  losses.  The  pistons  must  make  their  strokes 
together  in  such  engines,  but  they  may  move  in  the  same 
direction,  as  shown  in  the  figure,  or  in  opposite  directions. 

The  ideal  diagram  would  be  that  shown  at  (a)  by  the 
lines  AbcdCDA.  The  idealized  high-pressure  diagram  is 
abcda  and  the  idealized  low-pressure  diagram  is  ABCDA. 
The  exhaust  line  da  of  the  high-pressure  diagram  and  the 
admission  line  BC  of  the  low-pressure  diagram  are  pro- 
duced at  the  same  time.  Corresponding  points  on  these 
two  lines  represent  the  common  pressures  assumed  by  the 
steam  not  yet  exhausted  from  the  high-pressure  cylinder, 
the  steam  in  the  small  connecting  passage  and  the  steam 


COMPOUNDING 


151 


already  admitted  to  the  low-pressure  cylinder.  As  the 
movement  of  the  low-pressure  piston  opens  up  volume 
faster  than  the  high-pressure  piston  closes  up  volume, 
the  volume  occupied  by  the  steam  continues  to  increase  as 
the  low-pressure  piston  moves  out,  and  its  pressure  there- 
fore decreases. 

The  two  diagrams  are  shown  back  to  back  at  (6)  in  the 
figure  and  the  horizontal  line  xX  connects  corresponding 


FIG.  92. 

points  on  the  exhaust  of  the  high  pressure  and  the  admission 
of  the  low  pressure. 

Compound  engines  are  also  divided  into  two  types 
on  the  basis  of  cylinder  arrangement.  When  the  axes  of 
both  cylinders  coincide  as  shown  in  Fig.  92  they  are  called 
tandem  compounds.  When  the  axes  are  parallel  as  shown 
in  Fig.  89,  the  engines  are  spoken  of  as  cross-compound 
engines. 

74.  Cylinder  Ratios.  The  idealized  diagrams  of  a  com- 
pound engine  with  infinite  receiver 
volume  are  shown  in  Fig.  93  by 
abed  and  ABODE.  The  height  of 
the  high-pressure  exhaust  line  is  the 
same  as  that  of  the  low-pressure 
admission  line  and  represents  the 
receiver  pressure  PR.  The  value  of 
the  receiver  pressure  is  determined 
by  the  point  chosen  for  cut-off  in  the  low-pressure  cylinder. 
Thus  if  cut-off  in  the  low-pressure  cylinder  is  made  to  occur 
earlier,  as  at  some  point  c',  the  admission  line  for  this 


FIG.  93. 


152 


STEAM  POWER 


cylinder  must  move  up  to  B'C'  and  the  receiver  pressure 
must  rise  correspondingly.  The  exhaust  pressure  in  the 
high-pressure  cylinder  would  also  rise  an  equal  amount. 

Changing  the  point  of  cut-off  in  the  low-pressure  cylinder 
also  produces  another  result.  As  the  receiver  pressure  rises 
the  work  area  of  the  high-pressure  diagram  is  obviously  de- 
creased, while  that  of  the  low-pressure  diagram  is  increased. 
In  a  simple  engine  the  area  of  the  diagram  becomes  smaller 
the  earlier  the  cut-off,  and  it  should  be  noted  that  just  the 
reverse  of  this  occurs  in  the  low-pressure  cylinder  of  a  com- 
pound engine. 

It  is  evident  that  the  choice  of  the  receiver  pressure  or 
of  the  point  of  cut-off  in  the  low-pressure  cylinder  determines 
the  relative  areas  of  the  high-pressure  and  low-pressure 
diagrams  and  it  also  determines  the  relative  size  of  the  two 
cylinders.  The  diagram  of  Fig.  93  shows  that  late  cut-off 
in  the  low-pressure  cylinder  calls  for  a  larger  high-pressure 
cylinder  than  does  early  cut-off. 

The  ratio  of  the  piston  displacement  of  the  low-pressure 
cylinder  to  that  of  the  high-pressure  cylinder  is  called  the 
cylinder  ratio.  Designating  this  ratio  by  R,  and  using  other 
symbols  as  in  Fig.  93, 


(61) 


The  cylinder  ratios  chosen  for  real  compound  engines 
vary  greatly  in  different  designs  and  no  given  ratio  has  been 
proved  the  best  for  a  given  set  of  conditions.  Normal 
practice  gives  the  average  values  listed  in  Table  VI,  but 
cylinder  ratios  as  high  as  7  have  been  used  with  excellent 
results. 

TABLE  VI 

CYLINDER  RATIOS  FOR  COMPOUND  ENGINES 


Cylinder  ratio  

2f 

3| 

4 

^ 

Initial  pressure  (gauge)  non-condensing  

100 

120 

Initial  pressure  (gauge)  condensing  

100 

120 

150 

COMPOUNDING  153 

The  cylinder  ratio  to  be  used  in  a  given  case  may  be 
determined  by  any  one  of  several  considerations  or  by  a 
combination  of  them,  the  latter  being  more  often  the  case. 
Thus  it  may  be  deemed  desirable  to  obtain  the  same  amount 
of  work  from  both  cylinders;  or  to  obtain  equal  temperature 
ranges;  or  to  have  cut-offs  occur  at  the  same  fraction  of 
the  strokes;  or  to  have  the  same  total  load  on  the  two 
piston  rods  during  admission;  or  to  obtain  the  maximum 
possible  uniformity  of  turning  effort  at  the  crank.  The  con- 
sideration of  equal  work  is  generally  , 
regarded  as  the  most  important.  p^ 

75.  Indicator  Diagrams  and  Mean   ^ 
Pressures.      The   idealized    diagrams  p 
for  a  compound  engine  with  clearance, 
with    incomplete    expansion   in    both  p 
cylinders,    arid    without    compression 
are  given   in  Fig.  94.     The    nominal 

total  ratio  of  expansion  would  be  LL+1H,  but  the  total  ratio 
of  expansion  taking  account  of  clearance  is 

T     I  r<] 
Total  ratio  of  expansion  ==    L .  ~7L,      .     .     (62) 


CL 


and  the  cylinder  ratio  is 

R  =  ~- (63) 

The  mean  effective  pressures  can  be  found  from  each 
of  the  diagrams  in  the  ordinary  way  and  the  indicated 
horse-power  of  each  cylinder  determined  therefrom.  The 
indicated  horse-power  of  the  engine  is  then  equal  to  the  sum  of 
the  values  obtained  for  the  separate  cylinders. 

It  is  often  convenient  to  refer  the  mean  effective  pres- 
sure of  all  cylinders  to  the  low-pressure  cylinder  as  though 
this  were  the  only  cylinder  acting.  In  the  simple  form 
of  diagram,  such  as  that  shown  in  Fig.  93,  it  is  obvious 
that  this  could  be  obtained  by  measuring  the  area  AbcDEA, 


154  STEAM  POWEE 

dividing  by  the  length  AE  and  multiplying  by  the  scale  of 
the  spring,  just  as  though  the  diagram  were  all  produced 
in  one  cylinder  with  the  piston  displacement  equal  to  VL. 
In  the  case  of  the  diagrams  given  in  Fig.  94  a  similar  method 
could  be  adopted,  or  the  mean  effective  pressure  of  each 
cylinder  could  be  determined  separately  and  then  the 
equivalent  pressure  which  would  give  the  same  result  on 
the  low-pressure  piston  could  be  determined  analytically. 

Assume  for  this  purpose  that  the  mean  effective  pressure 
of  the  high-pressure  is  equal  to  pH  pounds  per  square  inch, 
that  the  mean  effective  pressure  of  the  low-pressure  cylinder 
is  equal  to  pL  and  that  the  cylinder  ratio  is  R.  The  strokes 
of  all  cylinders  of  a  multi-expansion  engine  are  generally 
equal,  so  that  the  piston  areas  are  in  the  same  ratio  as  the 
cylinder  volumes  (piston  displacements).  In  the  case  of 
a  2x  engine,  therefore,  the  area  of  the  low-pressure  piston 
is  R  times  as  great  as  that  of  the  high-pressure  piston, 
and  the  pressure  required  on  the  low-pressure  piston  to  do 
the  same  work  as  that  done  by  pressure  pn  on  the  high- 

pressure  piston  will  be  ^. 
R 

In  the  case  of  a  2x  engine  therefore  the  total  M.E.P. 
referred  to  the  low-pressure  cylinder  is 

(64) 


This  mean  effective  pressure  acting  on  the  low-pressure 
piston  only  would  give  the  same  indicated  horse-power  as 
is  obtained  with  the  two  cylinders  of  the  engine. 

In  designing  compound  engines  it  is  customary  to 
determine  the  size  of  the  low-pressure  cylinder  as  though  it 
were  to  do  all  the  work  expected  of  the  engine  by  receiving 
steam  at  the  highest  pressure  available  and  exhausting  it 
at  the  lowest.  The  mean  effective  pressure  which  would 
thus  be  assumed  to  exist  is  the  referred  value  PR  just  ex- 
plained. Having  found  the  size  of  the  low-pressure  cylinder. 


COMPOUNDING  155 

and  the  value  of  the  referred  M.E.P.  the  size  of  the  high- 
pressure  cylinder  can  be  determined  so  that  the  work  done 
by  each  cylinder  will  be  just  half  of  the  total  for  which  the 
engine  is  being  designed.  This  size  will  have  to  be  such 
that  the  high-pressure  mean  effective  pressure  referred  to 
the  low-pressure  cylinder  (i.e.,  pH  +  R)  is  equal  to  half  the 
total  mean  effective  pressure  referred  to  that  cylinder. 
That  is,  the  size  will  have  to  be  so  chosen  that 


ILLUSTRATIVE  PROBLEM 

A  double-acting  compound  engine  is  capable  of  developing 
500  I.h.p.  The  stroke  is  18  ins.;  revolutions  per  minute,  175; 
mean  effective  pressure  referred  to  L.P.  piston,  45  Ibs.  per  square 
inch;  cylinder  ratio,  3^.  Find  cylinder  diameters. 

From 


"33,000' 

500X33,000 


«L.P.  = 


45X1.5X175X2 


so  that 

dL.p.  =\r^7  =30  ins.  (approx,), 

\  ./oO 

with  the  cylinder  ratio  equal  to  3j, 
700 


/200 
dH.p.  =  *fogg  =  16  ins.  (approx.). 

76.  Combined  Indicator  Diagrams.  When  a  compound 
engine  is  indicated,  the  diagrams  of  the  two  cylinders  as 
drawn  by  the  indicator  are  not  directly  comparable.  The 
scales  of  pressure  and  volume  are  different  on  the  two  dia- 
grams, and  correction  must  be  made  for  this  fact  before  the 


156 


STEAM  POWER 


diagrams  can  be  compared.  It  is  customary  to  do  this  and 
to  draw  the  average  high-pressure  and  low-pressure  diagrams 
on  the  same  set  of  coordinates  in  order  to  determine  how 
well  they  approximate  the  ideal  diagram  that  would  be 
obtafned  in  one  cylinder  operating  between  the  extreme 
limits  of  pressure. 

Diagrams  approximating  those  that  would  be  obtained 
from  high-  and  low-pressure  cylinders  are  shown  at  (h) 
and  (7)  respectively,  in  Fig.  95,  and  the  result  of  drawing 


b        s  I  ....  I  ...  .1  i ....  i  .... i 


FIG.  95. 

both  to  the  same  scales  is  shown  at  the  left  of  this  figure. 
The  curves  Xh  and  Xi  show  the  variations  of  quality  along  the 
two  expansion  curves. 

Drawing  the  two  diagrams  to  the  same  scales  in  this 
way  is  known  as  combining  the  diagrams  and  the  result  is 
known  as  a  combined  diagram. 

The  curves  SS  and  S'S'  added  to  the  combined  diagram 
are  saturation  curves.  They  do  not,  in  general,  form  a 
continuous  curve,  because  of  the  different  quantities  of 
steam  contained  in  the  two  clearances  and  because  any 


COMPOUNDING 


157 


moisture  in  the  high-pressure  exhaust  is  generally  removed 
in  the  receiver.  The  volumes  occupied  by  clearance  steam 
at  initial  pressures  are  indicated  by  the  points  6'  and  B' 
respectively.  The  lengths  b'S  and  B'S'  approximately 
represent  the  volumes  that  would  be  occupied  by  cylinder 
feed  when  in  each  cylinder  if  dry  and  saturated. 

A  combined  diagram  for  a  triple-expansion  engine  is 
shown  in  Fig.  96.  The  heavy  lines  give  diagrams  con- 
structed so  as  to  represent 
as  nearly  as  possible  what 
may  be  expected  to  occur  in 
the  cylinders  of  such  an  en- 
gine, assuming  perfect  valve 
action  and  hyperbolic  expan- 
sions and  compressions.  The 
dotted  diagrams  indicate  the 
shapes  that  would  be  drawn 
by  indicators  applied  to  the 
real  cylinders.  The  numer- 
ous sharp  angles  are  due  to 
overlapping  of  events,  one 
cylinder  suddenly  starting  to  draw  from  a  receiver  while 
another  is  exhausting.  It  will  be  observed  that  the  dotted 
diagrams  do  not  contain  any  of  these  sharp  angles,  but 
that  their  general  outline  forms  a  fair  average  of  them. 

The  curve  cd  is  a  rectangular  hyperbola  drawn  as  a 
continuation  of  the  assumed  hyperbolic  expansion  line  of 
the  high-pressure  cylinder.  The  failure  of  the  expansion 
lines  of  the  other  cylinders  to  fall  upon  this  curve  is  ex- 
plained by  quality  changes,  different  quantities  of  clearance 
steam  in  the  different  cylinders  and  withdrawal  of  moist- 
ure from  steam  exhausted  to  receiver  before  admission  to 
the  following  cylinder, 


-- --d 


FIG.  96. 


158  STEAM  POWEE 


PROBLEMS 

1.  Find  the  size  of  the  cylinders  of  a  double-acting  compound 
engine,  which  is  to  give  600  I.h.p.,  when  using  steam  at  a  pressure 
of  150  Ibs.  per  square  inch  absolute,  and  having  a  back  pressure 
of  2  Ibs.  per  square  inch  absolute.     The  cylinder  ratio  is  to  be  4, 
and  the  total  ratio  of  expansion  12,  piston  speed  750  ft.  per  minute, 
and  R.P.M.  =150;  diagram  factor  is  80%. 

2.  Given  a  200  H.P.   compound  Corliss  engine  with  cut-off 
in  the  H.P.  cylinder  at  60%  stroke.     Ratio  of  expansion  is  7; 
clearance  is  7%;    card  factor  is  70%;    pressure  at  the  H.P.  cyl- 
inder is  165  Ibs.  absolute.     Find 

(a)  Cylinder  ratio;  V»  3 

(6)  Theoretical  and  actual  M.E.P.; 

(c)   Determine  size  of  four  engines,  and  select  the  best  one. 


3.  Given  a  compound  engine  18X40  ins.,  having  a  stroke  of 
28  ins.  Steam  pressure  is  165  Ibs.  per  square  inch  absolute; 
cut-off  in  H.P.  cylinder  occurs  at  62%  stroke;  clearance  equals 
16%;  back  pressure  equals  5  Ibs.;  R.P.M.  equal  150.  Find 

(a)  Cylinder  ratio;          4.4</  -.   y 

(6)  Ratio  of  expansion;      7.V 

(c)  Actual  M.E.P.;     &i 

(d)  Lh.p.    mo 


CHAPTER  X 
THE   D-SLIDE   VALVE 

77.  Description  and  Method  of  Operation.  The  simple 
D-slide  valve,  shown  in  place  in  Fig.  97,  is  so  named  because 
of  the  similarity  of  its  section  to  the  letter  D.  It  is  located 
in  the  steam  chest,  rides  back  and  forth  upon  its  seat  and 


FIG.  97. 

serves  to  connect  the  two  ports  alternately  with  steam 
and  exhaust  spaces  respectively  in  order  to  give  the  neces- 
sary distribution  of  steam. 

The  valve  has  to  perform  the  following  functions  for 
each  end  of  the  cylinder  during  each  revolution  of  the 
engine : 

(1)  It  connects  the  proper  port  to  the  steam  space  or 

159 


160  STEAM  POWER 

steam  chest  at  such  a  time  that  steam  can  enter  the  cylinder 
as  the  piston  moves  away  from  the  head. 

(2)  It  shuts  off  this  port  and  thus  cuts  off  the  supply 
of  steam  when  the  piston  has  completed  a  certain  definite 
fraction  of  the  stroke. 

(3)  It  connects  the  port  with  the  exhaust  cavity  shortly 
before  the  piston  reaches  the  end  of  the  stroke,  thus  effecting 
"  exhaust  "  or  "  release  ";   and 

(4)  It  shuts  off  the  port  again  when  the  piston  has  com- 
pleted the  proper  fraction  of  the  next  stroke,  thus  trapping 
in   the    cylinder   the   steam   which    is    compressed    during 
the  remainder  of  the  stroke. 

Engine  Crank 
Main  Connecting 


It  is  obvious  that  the  valve  must  be  reciprocated  upon 
its  seat  and  that  its  motion  must  be  connected  with  that  of 
the  piston  in  some  way  so  that  the  proper  phase  relation 
may  be  retained.  This  could  be  effected  by  the  system 
shown  diagrammatically  in  Fig.  98,  a  small  crank  operating 
on  the  end  of  a  connecting  rod  giving  the  valve  its  short 
stroke  just  as  the  main  crank  fixes  the  longer  stroke  of 
the  piston.  Such  an  arrangement  would,  however,  be 
very  inconvenient  with  many  real  engines,  as  the  valve  would 
be  located  too  far  from  the  center  line  of  the  cylinder. 

It  is  customary  to  use  what  is  known  as  an  eccentric 
for  the  purpose  of  operating  the  slide  valve.  The  parts 
and  arrangement  of  an  eccentric,  together  with  an  illus- 
tration of  the  wav  in  which  it  is  mounted  on  the  shaft  of 


FIG.  99. — Parts  of  Eccentric. 


162 


STEAM  POWER 


THE  D-SLIDE  VALVE 


163 


an  engine  are  shown  in  Figs.  99,  100  and  101.  The  motion 
it  gives  the  valve  is  exactly  the  same  as  that  imparted 
by  the  crank  first  assumed,  and  it  can 
easily  be  shown  that  it  is  the  exact 
equivalent  of  such  a  crank. 

Assume,  for  example,  a  crank  such 
as  that  shown  in  Fig.  98  with  a  length 
of   arm   or   throw  equal  to  a.     If  the 
crank   pin  is  made  larger  while  other 
parts  of  the  crank  remain  the  same,  as 
shown  in    Fig.    102,  the   crank  mech-    FlG-  101. -Eccentric 
anism  is  not  essentially  altered;  the  mo- 
tion  which   it  would   impart  to  a  connecting   rod  is  not 
changed.     If  this  process  of  enlarging  the  pin  be  continued 


1 
FIG.  102. — Equivalence  of  Crank  and  Eccentric. 


until  the  pin  has  become  large  enough  to  surround  the 
shaft  and  if  the  crank  arm  be  then  removed  so  that  what 
was  the  crank  pin  is  fastened  directly  on  the  shaft,  an 


164 


STEAM  POWER 


FIG.  103. 


FIG.  104. — Slide  Valve  without  Lap, 


team  Space 

Exhaust  Cavity  Connected 
to  Exhaust  Pipe 


Valve  and  piston  both  start 
to  move  toward  right. 


Piston  at  mid-stroke,  valve 
at  end  of  stroke  and  about 
to  return. 


illj  \  Valve  just  closing  left  hand  steam 
port  and  about  to  open  right  hand 
port  to  steam.  Piston  about  to  start 
on  return  stroke. 


Piston  at  mid-stroke,  valve 
wide  open  and  about  to  return. 


FIG.  105. 


THE  D-SLIDE  VALVE 


165 


eccentric  results.  It  is  the  exact  equivalent  of  the  original 
crank;  its  center,  which  is  the  center  of  the  crank  pin, 
revolves  about  the  center  line  of  the  shaft  in  a  circle  with 
a  radius  a  just  as  in  the  original  mechanism. 

The  eccentric  makes  it  possible  to  place  a  short  crank 
(short  arm)  upon  a  large  diameter  shaft  without  having  to 
cut  the  shaft  away  as  shown  in  Fig.  103,  and  it  is  therefore 
very  useful  for  driving  valves. 

78.  Steam  Lap.  The  simplest  possible  form  of  D-slide 
valve  would  just  reach  the  outer  edges  of  the  ports  when 
in  its  central  position  as  shown  in  Fig.  104.  The  crank 
driving  it  (that  is  the  crank  equivalent  to  the  eccentric 
which  would  probably  be  used  in  a  real  case)  would  have 
to  be  located  90°  ahead  of  the  engine  crank  in  the  direction 
of  rotation,  as  can  easily  be  seen  by  consulting  Fig.  105, 
which  illustrates  the  mechanism  in  various  critical  positions. 
The  illustration  shows  that  such  a  valve  would  give  full 
stroke  admission,  thus  producing  a  rectangular  cycle  which 
has  already  been  shown  to  be  very  inefficient  as  a  means  of 
obtaining  work  from  the  heat  .used  in 
forming  steam. 

If  cut-off  is  to  occur  before  the 
end  of  the  stroke,  the  edge  of  the 
valve  must  return  and  close  the  port 
before  the  piston  reaches  the  end  of  its 
stroke.  But  since  the  crank  mechan- 
ism does  not  permit  the  valve  to 
remain  stationary  in  any  one  posi- 
tion, such  early  cut-off  could  only  occur  if  the  valve 
over-traveled,  as  shown  in  Fig.  106,  and  this  would  un- 
fortunately result  in  connecting  the  working  end  of  the 
cylinder  to  exhaust  and  in  admitting  steam  to  the  other 
side  of  the  piston  at  such  a  time  as  to  oppose  the  piston's 
motion.  The  solution  of  the  difficulty  lies  in  making 
the  valve  longer,  so  that  when  in  its  central  position  it 
overlaps  the  outer  edges  of  the  ports  as  shown  in  Fig.  107. 


Steam  Space 

-« — Valve  Travel 


Exhaust/ 
Cavity 


Piston  Travel 


FIG.  106. 


166 


STEAM  POWER 


The  amount  of  overlap  of  the  outer  edge  is  called  the  out- 
side lap,  and  when  steam  is  admitted  by  the  outer  edges  of 
the  valve,  as  in  the  case  under  discussion,  it  is  also  called 
the  steam  lap. 

With  such  an  arrangement  the  valve  must  be  drawn 
out  of  its  central  position  by  the  amount  of  the  lap  when 
the  piston  is  at  the  end  of  its  stroke  as  shown  by  a  in  Fig. 


il 


FIG.  107. — Steam  and 
Exhaust  Lap. 


FIG.  108. — Lap  a  and  Lap 
Angle  a. 


108  in  order  that  steam  may  be  admitted  just  as  the 
piston  starts  to  move.  It  follows  that  the  crank  driving 
the  valve  must  be  more  than  90°  ahead  of  the  engine 
crank  and  that  it  must  be  ahead  by  The  angle  required 
to  move  the  valve  a  distance  equal  to  the  outside  lap. 
This  angle,  represented  in  the  figure  by  a,  is  called  the  lap 
angle. 

79.  Lead.  In  real  engines  it  is  further  desirable  to  start 
the  admission  of  steam  just  before  the  piston  arrives  at 
the  end  of  its  stroke.  This  assists  in  bringing  the  moving 
parts  to  rest,  raises  the  pressure  in  the  clearance  to  full 
value  before  the  piston  starts,  and  gives  a  wider  opening 
through  which  the  steam  can  flow  during  the  early  part  of 
the  stroke,  thus  reducing  wiredrawing  and  loss  of  area  at 
the  top  of  the  diagram.  If  the  valve  is  to  open  before  the 
piston  reaches  the  end  of  its  stroke,  the  crank  driving  it 
must  be  shifted  still  further  ahead  of  the  engine  crank. 
It  must  be  shifted  ahead  by  an  angle  which  will  draw 
the  valve  through  the  distance  which  will  give  the  desired 
opening  of  valve  with  the  piston  at  the  end  of  its  stroke 
as  shown  by  6  in  Fig.  109.  The  angle  required,  indicated 


.  THE  D-SLIDE  VALVE  167 

by  ft,  is  known  as  the  angle  of  lead,  and  the  width 
of  the  steam  opening  with  engine  crank  on  dead  center, 
i.e.,  the  distance  b,  is  known  as  the  lead.  The  lead 
varies  from  less  than  ^  in.  on  small  engines  and  with  low 
speeds  up  to  over  f  in.  on  large  engines  and  with  very  high 
speeds. 

80.  Angle  of  Advance.  The  eccentric  or  valve-operating 
crank  must  be  ahead  of  the  engine  crank  by  an  angle  equal 
to  90°+angle  of  lap  a+angle  of  lead  0,  as  can  be  seen 


FIG.  109.— Lead  6  and  Lead  Angle  (3. 

by  an  inspection  of  Fig.  109.  The  sum  of  a  and  ft  is  called 
the  angle  of  advance  and  will  be  represented  by  5.  This 
is  the  number  of  degrees  in  excess  of  90  by  which  the  eccen- 
tric leads  the  engine  crank. 

Fig.  109  shows  that  cut-off  in  an  engine  fitted  with 
a  valve  having  lap  and  lead  must  occur  when  the  engine 
crank  has  turned  through  an  angle  equal  to  180  — 2«,  because 
the  valve  will  then  have  returned  to  the  closed  position. 
Apparently,  cut-off  can  be  made  to  occur  at  any  point 
in  the  stroke  by  properly  choosing  the  value  of  a,  but  it 
will  be  discovered  later  that  the  exhaust  events  set  a  limit 
to  increase  in  the  value  of  this  angle  and  hence  do  not  per- 
mit of  cut-off  occurring  earlier  than  a  certain  fraction  of  the 
stroke. 

81.  Exhaust  Lap.  Inspection  of  Fig.  105  will  show  that 
the  simple  valve  without  lap  originally  assumed  will  give 
no  compression,  because  the  cylinder  end  is  connected  to 
the  exhaust  cavity  for  the  entire  stroke.  Inspection  of  all 


168  STEAM  POWER 

the  changes  which  have  been  suggested  in  the  subsequent 
paragraphs  will  show  further  that  if  the  inner  edges  of  the 
valve  are  left  in  the  original  positions  the  exhaust  events 
will  be  considerably  distorted  in  the  case  of  a  valve  having 
steam  lap  and  lead. 

This  trouble  may  be  remedied  by  moving  the  inner 
edges  of  the  valve  closer  together,  making  the  exhaust 
cavity  in  the  valve  shorter  and  giving  inside  lap  as  shown 
in  Fig.  107  by  6.  When  the  inner  edges  of  the  valve  control 
exhaust,  as  in  the  case  of  the  valve  under  discussion,  this 
inside  lap  is  also  called  exhaust  lap. 

The  length  of  the  valve,  the  lap  and  the  lead  are  gen- 
erally  chosen   so   as   to   give   the   desired   arrangement   of 
admission  and  cut-off  and  then  the  exhaust  edges  are  so 
located  as  to  give  desirable  release  and 
compression.    In  some  forms  this  necessi- 
tates the  use  of  an  exhaust  cavity  in  the 
FIG.  110.  valve   such   as  that   shown   in  Fig.  110. 

The  amount  by  which  the  edges  of  the 
valve  fail  to  meet  the  inner  edges  of  the  port  is  spoken  of  as 
negative  inside  lap.  This  dimension  is  indicated  by  c  in  the 
figure. 

It  should  be  noted  particularly  that  all  measurements 
of  lap  are  made  with  the  valve  central  on  its  seat  and 
that  the  measurement  of  lead  is  made  with  the  piston  at 
the  end  of  its  stroke,  i.e.,  with  the  engine  crank  on  dead 
center. 

82.  The  Bilgram  Diagram.  The  action  of  all  slide 
valves  could  be  studied  by  means  of  drawings  of  the  actual 
mechanism,  as  has  been  done  in  preceding  paragraphs,  but 
such  a  method  is  time  and  space  consuming.  Numerous 
diagrams  such  as  the  Elliptical,  the  Sweet,  the  Zeuner  and 
the  Bilgram  have  been  developed  for  the  purpose  of  simpli- 
fying and  expediting  such  studies  and,  when  properly 
understood,  they  are  very  convenient.  The  scope  of  this 
book  does  not  permit  a  discussion  of  all  of  these  diagrams 


THE  D-SL1DE  VALVE 


169 


and,  since  the  Bilgram  diagram  is  probably  the  most  gener- 
ally applicable,  attention  will  be  confined  to  it. 

The  construction  of  this  diagram  is  illustrated  in  Fig. 
111.  The  point  0  represents  the  center  of  the  engine 
crank  shaft  and  the  two  circles  drawn  about  this  point 
as  a  center  represent  respectively  the  paths  traveled  by  the 


FIG.  111. 


pin  of  the  valve  crank   and  the  pin  of  the  engine  crank. 
These  circles  are  drawn  to  any  conven:ent  scales. 

The  diagram  is  conventionally  drawn  in  such  a  way 
that  the  line  OM  represents  the  head  end  dead  center 
position  of  the  crank  and  in  all  subsequent  paragraphs  the 
relative  positions  shown  by  the  small  sketch  in  Fig.  Ill 
will  be  assumed.  The  cylinder  will  be  assumed  to  the 


170  STEAM  POWER 

left  of  the  shaft  and  the  engine  will  be  assumed  to  run 
"  over." 

With  the  crank  in  position  OM,  the  eccentric  (equivalent 
crank)  must  be  in  the  position  OB,  ahead  of  the  crank  by 
an  angle  90°-|-«+0  =  90+<5.  The  valve  must  then  be 
displaced  to  the  right  of  its  central  position  by  an  amount 
represented  by  the  distance  DB,  if  a  small  correction  for 
"  angularity  "  of  the  valve  connecting  rod  be  neglected. 
As  rotation  continues,  horizontal  distances  corresponding 
to  this  line  will  always  give  the  instantaneous  valve  dis- 
placements. For  position  OB' ,  for  instance,  the  valve  dis- 
placement will  be  D'B' '. 

If  the  angle  5  is  now  laid  off  above  OX,  locating  the  point 
Q  as  shown,  a  perpendicular  QE  dropped  upon  OX  from  this 
point  will  equal  in  length  the  line  DB,  and  will  therefore 
show  the  valve  displacement  when  the  crank  is  in  head  end 
dead  center  position  OM.  This  must  be  true,  because  the 
triangles  QOE  and  BOD  are  similar  and  have  the  sides 
OQ  and  OB  equal  to  the  radius  of  the  same  circle. 

The  perpendicular  QE  is  really  a  perpendicular  dropped 
upon  the  extension  of  the  line  representing  the  crank  posi- 
tion, and  it  is  a  general  property  of  this  diagram  that  a  line 
starting  at  Q  and  perpendicular  to  the  line  representing 
any  chosen  crank  position  (or  an  extension  of  that  line) 
will  show  by  its  length  the  displacement  of  the  valve  when 
the  crank  is  in  the  chosen  position.  Thus  assume  the  engine 
crank  to  rotate  through  the  angle  7  to  the  position  OM' . 
The  eccentric  will  have  rotated  to  B'  and  the  valve  dis- 
placement will  be  represented  by  D'B' .  A  perpendicular 
drawn  from  Q  upon  OX' ,  the  extension  of  the  crank  posi- 
tion, gives  QE  equal  to  B'D'  and  hence  representing  the 
valve  displacement  to  the  same  scale. 

This  construction  drawn  for  different  crank  positions 
OA,  OM,  OMi,  OM2,  etc.,  is  shown  in  Fig.  112,  the  dash- 
dot  radial  lines  about  Q  representing  the  various  values  of 
the  valve  displacement.  The  number  of  each  of  these 


THE  D-SLIDE  VALVE 


171 


lines  indicates  the  crank  position  to  which  it  corresponds. 
It  will  be  seen  that  the  displacement  increases  in  value 
until  the  crank  position  OM^  is  reached,  after  which  it 
decreases  again. 


Ma 


Steam  Lap 
Circle 


M, 


Since  the  opening 
ment  minus  the  lap,  as 
by  whicji  the  valve  is 
found  by  subtracting 
placement  the  amount 
head  end  dead-center 
to  lap  plus  lead,  and  is 


FIG.  112. 

to  steam  is  equal  to  the  displace- 
shown  in  Fig.  109,  the  actual  amount 
open  for  any  crank  position  can  be 
from  the  corresponding  valve  dis- 
of  lap  possessed  by  the  valve.  For 
position,  the  displacement  is  equal 
shown  by  QE  in  Fig.  112.  Subtract- 


172  STEAM  POWER 

ing  the  lead  EF,  the  remainder  FQ  gives  the  lap  of  the  valve. 
A  circle  drawn  about  Q  with  radius  equal  to  QF  (or  a  circle 
drawn  about  Q  and  tangent  to  the  line  L)  will  cut  off  of 
the  lines  representing  valve  displacement  the  amount 
representing  the  part  of  each  displacement  used  in  over- 
running the  lap  of  the  valve.  The  remainders,  that  is  the 
parts  of  the  lines  radiating  from  Q  in  Fig.  112  which  are 
outside  of  the  lap  circle,  must  then  represent  the  amounts 
by  which  the  valve  port  is  actually  open. 

It  will  be  observed  that  the  valve  is  open  by  the  amount 
of  the  lead  when  the  crank  is  on  dead  center,  position  OM . 
The  crank  position  for  which  the  valve  displacement  is  just 
equal  to  the  lap,  and  hence  at  which  the  valve  is  just  begin- 
ning to  open,  can  be  found  by  drawing  a  tangent  through 
0  to  the  lower  side  of  the  lap  circle  and  then  extending 
it  to  give  the  crank  position  OA  in  Fig.  112. 

As  the  crank  rotates  clockwise  from  this  position,  the 
valve  opens  wider  until,  when  position  OM^  is  reached, 
the  greatest  valve  opening  exists.  Further  rotation  results 
in  partial  closure  of  the  valve  and,  when  the  crank  has 
finally  rotated  into  position  OC,  the  valve  has  just  closed, 
that  is,  cut-off  has  occurred,  the  displacement  being  just 
equal  to  QG,  the  steam  lap. 

Thus  this  diagram,  as  so  far  developed,  indicates  crank 
positions  for  admission  and  cut-off  and  the  values  of  valve 
displacement  and  valve  openings  for  all  intermediate 
crank  positions. 

ILLUSTRATIVE    PROBLEM 

A  certain  valve  has  an  external  steam  lap  equal  to  1|  ins. 
The  lead  is  ^  in.  and  the  throw  of  the  eccentric  is  1\  ins.  (a)  Con- 
struct such  parts  of  the  Bilgram  diagram  as  are  necessary  to 
indicate  "head  end"  crank  positions  for  admission,  maximum 
valve  opening  and  cut-off.  (6)  Indicate  on  this  diagram  the 
amount  of  valve  opening  at  various  crank  positions  between 
admission  and  cut-off,  (c)  Determine  the  value  of  the  angle  of 
advance. 


THE  D-SLIDE  VALVE 


173 


Draw  a  circle  with  radius  equal  to  the  eccentric  throw,  2£ 
ins.,  using  any  convenient  scale.  This  circle  is  designated  by  abed 
in  Fig.  113.  Draw  about  the  same  center  another  circle  of  any 
convenient  size.  Draw  in.  the  horizontal  diameter  ac  and  extend 
as  shown.  On  the  right-hand  side  of  the  circle  draw  the  line  ef, 


Steam  Lap 
Circle 


FIG.  113. 


parallel  to  the  horizontal  axis  and  a  distance  above  it  equal  to  the 
lead,  iV  in.,  to  the  same  scale  as  that  chosen  for  eccentric  circle. 
The  steam  lap  circle  must  have  its  center  Q  on  the  upper  right- 
hand  quadrant  of  the  eccentric  circle,  and  it  must  be  tangent 
to  the  line  ef.  Its  radius  must  equal  the  steam  lap,  1J  in.  to  scale. 
Therefore,  with  compass  points  set  the  proper  distance  apart,  find 
the  center  Q,  about  which  a  l£-in.  radius  circle  will  just  be  tangent 
to  the  line  rf,  and  draw  the  steam  lap  circle. 


174 


STEAM  POWER 


The  crank  position  at  admission  is  found  by  drawing  the  line 
AO  so  that,  if  extended,  it  is  tangent  to  the  lower  side  of  the 
steam  lap  circle. 

The  crank  position  at  cut-off  is  found  by  drawing  the  line 


]  .^Inside  lap 


FIG.  114. 

M""0  in  such  position  that  it  is  tangent  to  the  upper  part  of 
the  steam  lap  circle. 

The  crank  position  for  maximum  valve  opening  is  found  by 
drawing  the  line  M"0  in  such  position  that  a  line  through  QO 
be  perpendicular  to  it.     The  amount  of  valve  opening  at  this 


THE  D-SLIDE  VALVE  175 

crank  position  is  shown  by  the  length  of  the  part  of  this  per- 
pendicular line  outside  of  the  steam  lap  circle,  i.e.,  the  distance 
Og  interpreted  according  to  the  scale  chosen  for  eccentric  and 
steam  lap  circles. 

When  the  crank  is  in  position  M' 0,  the  length  of  hi,  interpreted 
to  scale,  gives  the  amount  by  which  the  valve  is  open  to  steam. 

When  the  crank  is  in  position  M'"0,  the  length  of  jk,  inter- 
preted to  scale,  gives  the  amount  by  which  the  valve  is  open  to  steam. 

The  angle  indicated  by  5  is  equal  to  the  angle  of  advance 
because  of  the  property  upon  which  the  construction  of  this 
diagram  is  based. 

83.  Exhaust  and  Compression.  The  exhaust  edge  events 
can  be  shown  on  the  Bilgram  diagram  by  a  method  similar 
to  that  used  for  the  steam  edge  events.  The  direction  in 
which  valve  displacements  occur  are  indicated  in  the  upper 
part  of  Fig.  114  ih  which  the  crank  and  eccentric  circles 
have  been  drawn  to  such  scales  that  they  coincide.  In- 
spection of  the  small  sketch  in  the  lower  part  of  the  figure 
will  show  that  head  end  release  must  occur  when  the  valve 
has  traveled  a  distance  equal  to  the  inside  lap  to  the  left 
of  its  central  position.  A  crank  position  OR  drawn  tangent 
to  the  lower  part  of  a  circle  about  Q  with  radius  equal  to 
the  inside  lap  will,  therefore,  be  the  crank  position  at  re- 
lease. Clockwise  rotation  from  tbis  position  will  result 
in  a  wider  opening  to  exhaust  until  position  OM\  is  reached, 
after  which  the  valve  will  begin  to  close.  Final  closure 
will  occur  when  the  crank  reaches  position  OK,  the  exten- 
sion of  which  is  tangent  to  the  top  of  the  exhaust  lap  circle. 
At  that  time  the  valve  will  have  returned  (moving  from  left 
to  right)  and  will  still  have  to  move  a  distance  equal  to 
the  exhaust  lap  before  attaining  a  central  position. 

ILLUSTRATIVE   PROBLEM 

Given  the  exhaust  lap  of  a  D-slide  valve  equal  to  f  in.;  the 
steam  lap  \\  ins.;  the  throw  of  the  eccentric,  2  ins.;  and  the 
lead  |  in.  Find  the  angle  of  advance,  the  maximum  port  opening 
to  steam  and  to  exhaust,  and  the  crank  positions  of  cut-off,  release, 
compression  and  admission  for  the  head-end  of  the  cylinder. 


176 


STEAM  POWER 


Draw  the  eccentric  (and  crank)  circle  with  a  radius  equal 
to  2  ins.,  and  draw  the  horizontal  diameter  as  in  Fig.  115. 

Draw  a  horizontal  line  in  the  upper  right-hand  quadrant  at  a 
distance  of  f+lj  ins.  above  the  horizontal  diameter.  Locate 
the  point  Q  at  intersection. 


FIG.  115. 

Draw  the  steam  lap  circle  with  a  radius  lj  in.  and  the  exhaust 
lap  circle  with  a  radius  f  in. 

The  angle  of  advance  is  the  angle  between  OQ  and  the  hori- 
zontal. 

The  maximum  opening  to  steam  is  given  by  the  distance 
Oa=f  in.  The  maximum  opening  to  exhaust  is  given  by  the 
distance  Ob  =  If  in. 

The  crank  positions   shown   are   obtained  by   drawing  lines 


THE  D-SLIDE  VALVE 


177 


tangent  to  the  lap  circles.     A  represents  admission;    C,  cut-off; 
R,  release,  and  K,  beginning  of  compression. 

The  piston  positions  at  the  times  of  these  events  are  given 
to  reduced  scale  by  vertical  projection. 

84.  Diagram  for  Both  Cylinder  Ends.  The  complete 
diagram  for  the  head  end  cylinder  is  shown  in  Fig.  114  with 
all  critical  crank  positions  marked.  The  positions  for  the 
crank  end  of  the  cylinder  can  be  found  in  a  similar  way  by 
constructing  a  diagram  in  which  the  point  Q  and  the  lap 
circles  are  located  in  the  opposite  quadrant.  The  resulting 


E.  Admission 


FIG.  116. 

diagram  for  both  cylinder  ends,  with  laps  the  same  for  both 
ends  of  the  valve,  is  given  in  Fig.  116. 

85.  Piston  Positions.  The  valve  events  might  be  studied 
entirely  in  conjunction  with  crank-pin  positions,  but  it  is 
more  convenient  and  customary  to  consider  them  in  connec- 
tion with  piston  positions.  Piston  positions  corresponding 
to  different  crank-pin  positions  could  be  found  by  drawing 
the  mechanism  to  scale  for  each  different  position  as  shown 
in  Fig.  117  for  piston  positions  1  and  2. 

It  is  obvious  that  this  would  involve  a  great  deal  of  work 
and  that,  if  drawn  to  large  scale,  it  would  consume  a  great 


178 


STEAM  POWER 


deal  of  space.  Further,  it  is 
convenient  to  be  able  to  locate 
relative  piston  positions  on  the 
line  which  serves  as  the  hori- 
zontal diameter  of  the  crank 
circle  of  the  Bilgram  diagram. 
The  method  used  depends 
upon  the  fact  that  the  motion 
of  the  crosshead  is  exactly  the 
same  as  that  of  the  p'ston, 
so  that  if  the  motion  of  the 
crosshead  end  of  the  connect- 
ing rod  can  be  followed,  it 
will  be  equivalent  to  following 
the  motion  of  the  piston  itself. 
It  should  also  be  noted  that 
the  diameter  of  the  crank  cir- 
cle must  be  equal  to  the 
stroke  of  the  engine. 

Assume  now,  that  the  point 
b  in  Fig.  117  be  taken  to  rep- 
resent the  position  of  the  pis- 
ton when  it  is  really  in  posi- 
tion 1.  When  the  piston  has 
moved  to  position  2,  the  cross- 
head  will  have  moved  from  a 
to  a!  and  the  crank  pin  from 
6  to  6'.  If  with  a'  as  a  center 
the  connecting  rod  be  s  wung 
down  to  the  horizontal  its 
right-hand  end  will  arrive  at 
the  point  c.  The  distance  be 
must  then  represent  the  dis- 
tance that  crosshead  (and  pis- 
ton) have  moved  from  dead- 
center  position  because  ab  and 


THE  D-SLIDE  VALVE 


179 


a'c  both  represent  the  length  of  the  connecting  rod  and  c 
must  therefore  be  as  far  to  the  right  of  6  as  a'  is  to  the  right 
of  a.  The  point  c  may  therefore  be  taken  to  represent 
piston  position  when  the  connecting  rod  is  in  the  position 
a'b'. 

'  In  general,  if  the  horizontal  diameter  of  the  crank 
shaft  be  taken  to  represent  the  stroke  of  the  engine,  the  pis- 
ton position  corresponding  to  any  crank  position  can  be 
found  by  taking  a  radius  equal  to  the  connecting-rod  length 
(to  the  same  scale  as  the  circle)  and  striking  an  arc  from  the 


FIG.  118. 

crank-pin  position,  using  a  center  on  the  horizontal  line  on 
the  cylinder  side  of  the  crank  circle. 

An  approximate  method  is  also  used  for  finding  the  piston 
position.  Instead  of  projecting  down  from  the  crank-pin 
position  with  an  arc,  such  as  b'c  in  Fig.  117,  a  vertical  line 
through  the  crank-pin  position  is  used.  Such  a  line  would 
give  c'  as  the  piston  position  when  c  is  really  correct.  This 
method  would  give  accurate  results  with  a  connecting  rod 
of  infinite  length.  For  ordinary  lengths  of  rod,  however, 
the  results  are  far  from  correct.  The  error  is  said  to  be  due 
to  the  angularity  of  the  connecting  rod. 

The  effect  of  the  angularity  of  the  connecting  rod  is 
shown  in  Fig.  118  for  different  positions.  On  the  outstroke 
the  piston  is  always  farther  ahead  than  the  rectilinear  pro- 


180 


STEAM  POWER 


jection  would  indicate.     On  the  return  stroke  the  piston  is 
always  behind  the  position  indicated  by  rectilinear  projection. 


FIG.  119. 

86.  Indicator  Diagram  from  Bilgram  Diagram.      Since 
the  piston  positions   corresponding  to  different    crank  posi- 


THE  D-SLIDE  VALVE  181 

tions  can  be  determined,  it  is  a  comparatively  simple  matter 
to  construct  the  indicator  diagram  which  theoretically 
would  be  given  by  an  engine  fitted  with  a  valve  of  certain 
dimensions.  It  is  necessary  to  assume  the  upper  and  lower 
pressure  and  also  to  assume  the  form  of  the  expansion 
and  compression  curves.  These  are  generally  taken  as 
rectangular  hyperbolas. 

The  method  of  constructing  an  indicator  diagram  from 
the  Bilgram  diagram  is  shown  in  Fig.  119.  The  crank- 
pin  positions  for  admission  (A),  cut-off  (C),  release  (R) 
and  beginning  of  compression  (K)  are  first  found.  These 
pin  positions  are  then  projected  to  the  horizontal  diameter 
by  means  of  arcs  with  radius  equal  to  the  connecting-rod 
length  and  with  centers  on  the  line  MM  produced  to  the 
left.  The  intersections  a,  c,  r  and  k  indicate  the  piston 
positions  at  which  the  corresponding  events  occur.  These 
are  then  projected  vertically  downward  to  intersect  the 
proper  pressure  lines  and  the  card  is  drawn  through  the 
intersections. 

Diagrams  constructed  in  the  same  way,  but  for  both 
head  and  crank  ends,  are  given  in  Fig.  120.  A  symmetrical 
valve  was  assumed,  that  is,  one  built  exactly  alike  on  head 
and  crank  ends.  The  diagrams  show  that  such  a  valve 
cannot  give  the  same  results  for  both  cylinder  ends  because 
of  the  effect  of  the  angularity  of  the  connecting  rod.  It 
is  most  evident  in  the  case  of  cut-off.  The  cut-off  in  this 
case  occurs  just  before  three-quarter  stroke  for  the  head  end 
and  just  after  half  stroke  for  the  crank  end  of  the  cylinder. 
All  other  events  are  distorted  in  the  same  way,  but  the  actual 
lengths  of  the  variations  are  not  as  great  as  in  the  case  of 
the  cut-offs  and  therefore  the  distortion  is  not  as  obvious. 

The  effect  of  the  angularity  of  the  connecting  rod  upon 
the  diagrams  can  be  remembered  easily  if  it  is  noted  that  all 
valve  events  occur  later  with  respect  to  piston  position  on 
the  outstroke  and  earlier  on  the  instroke  than  they  would 
with  a  connecting  rod  of  infinite  length. 


182 


STEAM  POWER 


It  is  possible  to  "  equalize  "  the  cut-offs,  that  is,  make 
them  occur  at  the  same  fraction  of  the  stroke  by  using 
unequal  steam  laps  at  opposite  ends  of  the  valve,  but  this 
will  result  in  still  further  distortion  of  admissions,  as  can  be 
seen  by  constructing  a  Bilgram  diagram  for  this  case. 
Similarly,  the  compressions  can  be  equalized  by  the  use  of 


C.E.  Admission 


H.E.  Admissi 


JH.E}  Card  ir\          '$£•*• Card 


FIG,  120. 

unequal  exhaust  laps,  but  this  results  in  distortion  of  the 
release  events. 

Various  linkages  have  been  developed  which  are  so 
arranged  that  they  distort  the  motion  of  the  valve  to  just 
the  extent  necessary  to  counterbalance  the  effects  of  the 
angularity  of  the  connecting  rod.  The  scope  of  this  book 
does  not,  however,  permit  a  discussion  of  such  valve 
gears. 


THE  D-SLIDE  VALVE  183 

87.  Limitations  of  the  D-slide  Valve.  The  simple 
valve  discussed  in  the  preceding  paragraphs  has  numerous 
limitations  and  is  therefore  only  used  on  small  and  cheap 
engines,  or  in  cases  where  economy  in  the  use  of  steam  is 
not  essential.  This  valve,  when  used  with  steam  entering 
over  the  outside  edges  as  previously  considered,  is  pressed 
to  its  seat  by  the  live  steam  acting  over  its  entire  upper 
surface.  This  pressure  is  practically  unbalanced,  as  the 
greater  part  of  the  lower  surface  of  the  valve  is  subjected 
to  the  low  pressure  of  the  steam  being  exhausted.  As  a 
result  the  friction  to  be  overcome  in  moving  the  valve  is 
very  great  and  there  is  an  appreciable  loss  from  this  source. 

Further,  the  shape  of  the  valve  makes  necessary  the  use 
of  long  ports  which  form  part  of  the  cylinder  clearance 
and  which  are  alternately  exposed  to  live  and  to  exhaust 
steam  with  results  previously  discussed.  These  ports  can 
be  decreased  in  length  by  increasing  the  length  of  the  valve, 
but  this  in  turn  increases  the  area  exposed  to  high  pressure 
and  hence  increases  the  friction  loss. 

It  can  be  shown  by  means  of  the  Bilgram  diagram 
that,  if  a  cut-off  earlier  than  about  f  stroke  is  desired, 
the  angle  of  advance,  the  amount  of  steam  lap  and  the  size 
of  the  eccentric  must  all  be  made  very  great.  This  results 
not  only  in  large  friction  losses,  but  also  in  very  early  release 
and  compression,  because  of  the  great  angle  of  advance. 
As  a  result,  slide  valves  of  the  simple  D  type  are  seldom  used 
when  a  cut-off  earlier  than  |  to  ^  stroke  is  desired.  It 
should  be  remembered  in  this  connection  that  the  simple 
engine  generally  gives  its  best  economy  with  a  cut-off  of 
about  J  stroke. 

The  drawing  of  lines  representing  the  opening  of  the 
valve  to  steam  as  in  Fig.  112  will  show  that  this  simple 
valve  is  further  handicapped  by  the  very  slow  opening 
and  closing  of  the  steam  ports,  causing  a  great  amount  of 
wire  drawing  with  a  corresponding  loss  of  diagram  area. 
In  order  to  get  an  adequate  opening  to  steam  the  valve 


184 


STEAM  POWER 


must  also  be  given  a  great  displacement  and,  since  this 

occurs  under  great  pressure,  it  results  in  great  friction  loss. 

The   unbalanced  feature   can   practically  be   overcome 

by  rolling  up  the  valve 
and  ports  about  an  axis 
parallel  to  the  length  of 
the  cylinder.  This  gives 
what  is  known  as  a  pis- 
ton valve,  shown  dia- 
grammatically  in  Fig. 
121 

Jt    can    also    be 


-Steam  Ports 


FIG.  121.—  Piston  Valve. 

tially  overcome  by  using 

a  balance  plate  or  ring  of  some  kind  between  the  top  of 
the  valve  and  the  inside  of  the  steam-chest  cover,  so 
arranged  that  live  steam  is  excluded  from  the  greater  part 
of  the  upper  surface  of  the  valve.  Valves  of  this  type 
are  generally  called  balanced  slide  valves  and  are  used  on 
many  high-  and  medium-speed  engines. 

The  valve  travel  required  for  obtaining  a  given  opening 


Steam 


FIG.  122.— Allen  Double  Ported  Valve. 

can  be  decreased  and  the  rate  of  opening  and  closing  can  be 
increased  by  the  use  of  multiported  constructions.  These 
are  so  arranged  that  two  or  more  ports  open  or  close  at  the 
same  time,  so  that  the  total  movement  required  for  a  given 
opening  is  divided  by  the  number  of  ports  and  the  rate  of 
opening  and  closing  is  multiplied  in  the  same  proportion. 
One  simple  type  of  double-portea  valve  is  illustrated  in 
Fig.  122. 

When  several  ports  are  used  the  valve  often  becomes 


THE  D-SLIDE  VALVE  185 

a  rectangular  frame  crossed  by  a  number  of  bars  and  is 
known  as  a  gridiron  valve,  because  of  its  appearance.  Such 
valves  are  often  combined  with  balance  plates  and  give 
very  satisfactory  results. 

A  number  of  designs  of  slide  valves  have  been  developed 
for  the  purpose  of  making  cut-off  independent  of  the  other 
events.  Many  of  these  use  a  separate  cut-off  valve  which 
either  controls  the  steam  supply  to  the  main  valve  or  else 
rides  on  the  main  valve  and  controls  cut-off  by  covering 
ports  in  that  valve.  Devices  of  the  latter  type  are  called 
riding  cut-off  valves.  They  are  either  driven  by  separate 
eccentrics,  or  by  linkage  from  the  eccentric  controlling 
the  main  valve,  the  linkage  being  so  arranged  as  to  give 
the  proper  relative  motion  between  main  and  auxiliary 
valves.  In  such  designs  the  main  valve  is  proportioned 
so  as  to  give  the  desired  admission,  release  and  compres- 
sion and  the  cut-off  is  then  taken  care  of  by  proper  adjust- 
ment of  the  cut-off  valve. 

88.  Reversing  Engines.  It  was  shown  in  one  of  the  early 
paragraphs  of  this  chapter  that  the  eccentric  must  be  set 
90°+ angle  of  advance  ahead  of  the  crank,  ahead  meaning 
in  the  direction  of  rotation.  To  cause  the  engine  to  revolve 
in  the  opposite  direction,  that  is,  to  "  reverse  "  the  engine, 
it  is  therefore  only  necessary  to  shift  the  relative  positions 
of  eccentric  and  crank  so  that  the  eccentric  leads  the  crank 
by  90°  -\-5  in  the  new  direction  of  rotation.  This  corre- 
sponds to  shifting  ahead  (in  first  direction  of  rotation) 
through  an  angle  equal  to  180  —  25  or  shifting  backward 
through  an  angle  equal  to  180+26,  as  can  be  seen  by  inspec- 
tion of  Fig.  109. 

In  practice  it  is  generally  more  convenient  to  use  two 
eccentrics,  one  set  properly  for  rotation  in  one  direction 
and  the  other  set  properly  for  rotation  in  the  opposite  direc- 
tion. This  arrangement  is  shown  diagrammatically  in 
Fig.  123.  This  figure  is  drawn  for  a  vertical  engine  and  in 
such  position  that  the  engine  is  on  crank-end  dead  center. 


186 


STEAM  POWEE 


FIG.  123. 


Rever 


ight  Shaft 


The  point  P  represents  the  position  of  the  center  of  the  crank 

pin;  the  point  /  represents  the  position  of  the  equivalent 
crank  (center  of  eccentric)  which 
drives  the  valve  for  "  forward," 
"  ahead  "  or  clockwise  rotation;  and 
the  point  b  represents  the  position  of 
the  equivalent  crank  which  drives  the 
valve  for  "  backing/'  "  reverse,"  or 
counter-clockwise  rotation. 

The  real  mechanism,  in  one  of  its 
numerous  forms  known  as  the  Stephen- 
son  Link  Gear,  is  shown  in  perspective  in  Fig.  124.     The 

forward    eccentric    corresponds    to  /  of   Fig.   123  and  the 

backing  eccentric  corresponds 

to  6  of    that    figure.      The1 

eccentric  rods    are  fastened 

to  opposite  ends  of  a  curved 

"  link  "  and  move  the  valve 

through     a    "  link     block " 

fastened  to  the  end  of  the 

valve  stem.     In  the  position 

shown  in  the  figure  the  link 

is  in  such  position  that  the 

forward    eccentric    operates 

practically  directly    on    the 

valve  stem  so  that  the  valve 

motion  is  practically  entirely 

governed  by  that  eccentric. 

If  the  reverse  shaft  were  to 

be  rotated  clockwise  into  the 

backing  position,  the  "  sus- 
pension   rods "    would    pull 


Clock 
Rotat 


FIG.  124.— Stephenson  Link  Gear. 


the  link  over  until  the  eccen- 
tric rod  of  the  backing  eccentric  was  directly  under  the 
valve  stem.  Under  such  conditions  the  valve  motion 
would  be  controlled  almost  entirely  by  the  backing 


THE  D-SLIDE  VALVE  187 

eccentric  and  the  engine  shaft  would  rotate  counter-clock- 
wise. 

If  the  mechanism  were  so  set  that  the  link  block  occupied 
a  position  on  the  link  between  the  ends  of  the  two  eccentric 
rods,  the  valve  motion  would  be  controlled  by  both  eccentrics 
and  would  be  a  compromise  between  the  motions  given  by 
either  eccentric  separately.  It  is  characteristic  of  this  gear 
that  the  cut-off  is  latest  when  either  one  or  the  other  eccentric 
is  fully  "  in  gear  "  and  that  it  becomes  earlier  as  the  link 
block  approaches  the  center  of  the  link.  With  the  link 
block  in  the  center  of  the  link  the  valve  does  not  open  at 
all,  i.e.,  the  cut-off  occurs  at  zero  stroke. 

There  are  numerous  other  forms  of  link  gears,  the  best 
known  being  the  Gooch,  the  Allan  and  the  Porter-Allen. 
There  are  also  numerous  reversing  mechanisms  known  as 
radial  gears  in  which  the  motion  of  the  valve  is  controlled 
by  means  of  a  "  radius  rod  "  which  can  be  set  to  give  the 
desired  valve  motion.  The  valve  motion  is  obtained  in- 
directly through  the  radius  rod  from  an  eccentric,  from  the 
crank,  or  from  the  connecting  rod.  The  limits  of  this 
book  do  not  permit  a  detailed  discussion  of  these  forms. 

89.  Valve  Setting.  From  what  has  preceded  it  will 
be  evident  that  it  is  not  only  necessary  that  a  valve  and 
its  seat  and  driving  mechanism  be  correctly  designed,  but 
also  that  the  various  parts  must  be  correctly  connected  up 
in  order  that  the  valve  may  move  in  its  proper  phase  rela- 
tion with  respect  to  the  piston. 

Adjusting  the  mechanism  in  such  a  way  that  the  proper 
phase  relations  are  obtained  is  known  as  setting  the  valve. 
This  can  be  done  with  fair  accuracy  by  a  simple  study  of 
the  mechanism  in  various  positions,  as  will  be  shown  below, 
but  it  is  always  advisable  to  check  the  setting  by  means  of 
indicator  diagrams  taken  after  the  setting  is  completed. 
Such  diagrams  will  often  show  errors  of  such  character  or 
size  that  they  cannot  be  determined  by  measurement  on 
an  engine  which  is  not  operating. 


188 


STEAM  POWER 


Before  beginning  operations  it  is  always  advisable  to  go 
over  the  entire  engine  carefully  and  to  eliminate  excessive 
lost  motion  at  all  pins  and  bearings  in  order  that  the  relative 
positions  of  parts  obtained  while  setting  the  valve  may 
approximate  those  which  will  be  obtained  when  the  engine 
is  in  operation.  The  effect  of  lost  motion  will  be  appreciated 
after  a  study  of  Fig.  125.  Assume  that  all  parts  of  the 
mechanism  are  tight  except  the  crank-pin  end  of  the  con- 
necting rod  as  shown.  If  the  engine  is  rotated  by  hand, 


FIG.  125. 


for  instance -,  by  turning  the  fly-wheel,  the  crank  will  pull 
the  piston  mechanism  and  the  piston  will  be  drawn  into  the 
position  shown  in  the  upper  half  of  the  figure  when  the  crank 
has  turned  through  an  angle  a.  On  the  other  hand,  when 
the  engine  is  operating  under  steam,  the  piston  will  push 
the  crank  pin  around  and  will  occupy  a  position  such  as 
that  shown  in  the  lower  half  of  the  figure  when  the  crank 
has  been  turned  through  the  same  angle  a.  Obviously, 
the  piston  can  occupy  two  very  different  positions  for  the 
same  crank  position,  and  a  valve  setting  based  upon  the 
conditions  shown  in  the  upper  part  of  the  figure  might  be 


THE  D-SLIDE  VALVE  189 

very  incorrect  when  used  under  the  conditions  shown  in 
the  lower  part  of  the  figure. 

Lost  motion  in  any  part  of  the  mechanism  can  produce 
analogous  results  and  it  is  therefore  necessary  to  remove  as 
much  of  it  as  possible  before  attempting  to  set  the  valve. 
It  is  practically  impossible  to  eliminate  all  lost  motion,  as 
there  must  be  sufficient  clearance  at  all  bearing  surfaces 
to  accommodate  a  film  of  oil,  and  this  alone  would  make 
necessary  the  taking  of  indicator  diagrams  for  the  check- 
ing of  valve  settings,  even  if  it  were  possible  to  set  perfectly 
by  measurement  for  stationary  conditions. 

In  general,  there  are  two  adjustments  which  can  be  made 
in  setting  a  plain  slide  valve.  The  length  of  the  valve 
stem  or  eccentric  rod  can  be  changed  and  the  eccentric 
can  be  shifted  around  the  shaft.  It  is  necessary  to  under- 
stand the  effects  of  each  of  these  adjustments. 

Changing  the  length  of  the  valve  stem  is  equivalent  to 
shifting  the  valve  upon  its  seat  without 
moving  the  engine  as  shown  in  Fig. 
126.     In  this  figure  the  valve  is  shown 


in  its  central  position  by  full  lines.    The 
lap  is  the  same  at  both  ends.     If,  now,  FIG.  126. 

the  valve  is  worked  to  the  right  upon 
its  stem  by  adjustment  of  the  nuts  shown,  until  it  reaches 
the  dotted  position,  the  head-end  lap  will  have  been  de- 
creased and  the  crank-end  lap  will. have  been  increased  by 
the  same  amount.  This  would  make  admission  earlier  and 
cut-off  later  for  the  head  end  and  admission  later  and  cut- 
off earlier  for  the  crank  end.  Obviously,  the  effects  of 
changing  the  length  of  the  valve  stem  are  opposite  for  the 
two  ends  of  the  cylinder. 

Shifting  the  eccentric  about  the  shaft  simply  changes 
the  time  relation  between  valve  motion  and  piston  motion; 
it  does  not  alter  the  valve  motion  itself.  If  difficulty  is 
experienced  in  realizing  the  truth  of  this  statement,  it  is 
only  necessary  to  draw  several  Bilgram  diagrams  for  the 


190  STEAM  POWER 

same  valve,  but  with  different  angles  of  advance,  and  then 
to  construct  indicator  diagrams  for  both  cylinder  ends  in 
every  case.  It  will  be  discovered  that  shifting  the  eccentric 
ahead  in  the  direction  of  rotation,  for  instance,  will  make 
all  events  occur  earlier  with  respect  to  piston  position  for 
both  ends  of  the  cylinder. 

In  setting  a  plain  slide  valve  which  is  built  symmetrical 
about  a  central  axis,  i.e.,  same  inside  and  outside  lap  at 
each  end,  it  is  first  necessary  to  adjust  the  length  of  the  valve 
stem.  This  may  be  done  by  removing  the  steam-chest 
cover  so  as  to  expose  the  valve  and  then  rotating  the  engine 
slowly  by  hand  and  observing  the  distance  traveled  by  the 
valve  on  each  side  of  its  central  position.  This  is  con- 
veniently done  by  observing  the  distance  between  the  outer 
edge  of  the  steam  port  and  the  outer  edge  of  the  valve  when 
the  valve  is  fully  open  at  each  end.  If  the  valve  travels 
further  toward  the  head  end  than  it  does  toward  the  crank 
end,  with  reference  to  the  port  edges,  the  valve  stem  must 
be  shortened;  if  it  travels  further  toward  the  crank  end 
the  stem  must  be  lengthened. 

In  making  these  adjustments  it  is  advisable  to  turn  the 
engine  only  in  the  direction  in  which  it  is  going  to  rotate,  so 
that  any  lost  motion  in  the  valve  mechanism  will  have 
approximately  the  same  effect  as  when  the  engine  is  opera- 
ting. 

When  the  length  of  the  valve  stem  is  correctly  adjusted, 
the  eccentric  must  be  so  set  on  the  shaft  as  to  give  the  proper 
angle  of  advance.  This  is  commonly  done  by  shifting  it 
about  the  shaft  until  the  proper  value  for  the  steam  lead  has 
been  obtained.  In  order  to  determine  the  value  of  the  lead 
it  is  necessary  to  be  able  to  set  the  engine  on  each  dead  center. 
This  can  be  done  approximately  by  turning  the  engine  until 
the  crosshead  has  come  to  either  end  of  its  stroke,  but  it 
will  be  found  by  trial  that  the  fly-wheel  and  shaft  can  be 
turned  through  a  very  large  angle  at  each  end  of  the  stroke 
without  causing  an  appreciable  motion  of  the  crosshead, 


THE  D-SLIDE  VALVE 


191 


so  that  this  method  is  not  very  satisfactory  for  the  purpose 
of  adjusting  the  eccentric.  It  is  customary,  therefore, 
to  work  in  such  a  way  as  to  give  a  more  accurate  determina- 
tion of  shaft  and  crank  positions  for  dead  center. 

The  engine  is  rotated  until  the  crosshead  has  been 
brought  near  one  end  of  its  stroke,  as  shown  in  Fig.  127, 
and  a  mark  is  then  scribed  across  the  crosshead  and  guide 
as  at  ab.  An  arc  xy  is  then  marked  on  the  fly-wheel  by 
means  of  a  tram  such  as  that  shown,  the  end  c  being  placed 


FIG.  127. 

at  point  P  on  some  solid  part  of  foundation  or  floor.  The 
engine  is  then  rotated,  clockwise  in  the  figure,  until  the 
crosshead  has  reached  the  end  of  its  stroke  and  returned 
to  such  a  point  that  the  marks  on  crosshead  and  guides 
again  coincide,  as  shown  by  dotted  positions  in  the  figure. 
The  arc  x'y'  is  then  scribed  on  the  fly-wheel  with  the  tram, 
the  end  c  again  bearing  on  the  point  P.  A  point  z  is  then 
found  by  bisecting  the  arc  ef  and  when  this  point  is  brought 
under  point  d  of  the  tram  the  crank  will  obviously  be  at 
crank-end  dead  center  and  the  piston  at  the  crank  end 


192 


STEAM  POWER 


(a)  Perfect  Cards  for  Slide  Valve  Type. 


(6)  Actual  Card;  Small  Engine.  Center  Line  of  Valve  on  Center 
Line  of  Seat;  Eccentric  Advanced  to  Give  Normal  Lead  of 
0.05  inch.  Engine  Running  Over. 


(c)  Same  Setting  as  (6)  except  Engine  Running  Under. 
FIG.  128. 


THE  D-SLIDE  VALVE 


193 


(d)  Angular  Advance  of  Eccentric   Increased.     Valve  Stem  Length 
Same  as  in  (6)  and  (c),     Lead  0.375  Inch, 


0)  Angular  Advance  of  Eccentric  Decreased  so  as  to  Give  Negative 
Lead  of  0.5  Inch.    Length  of  Valve  Stem  Unchanged. 


C.  E. 


H.  E. 


(/)  Length  of  Valve  Stem  Changed;  Angle  of  Advance  as  in  (6). 

FIG.  128. 


!94  STEAM  POWER 

of  its  stroke.  A  point  on  the  fly-wheel  diametrically  opposite 
to  z  is  next  found,  so  that  when  it  is  brought  under  point 
d  of  the  tram  the  engine  will  be  on  head-end  dead  center. 

It  is  probable  that  more  accurate  results  are  obtained 
by  rotating  the  engine  in  a  direction  opposite  to  that  in 
which  it  rotates  under  steam,  because  lost  motion  is  then 
taken  up  in  the  same  direction  as  when  working,  but  when 
the  whole  process  of  valve-setting  is  considered  it  is  ques- 
tionable whether  this  is  the  correct  direction  of  rotation. 
Opinion  and  practice  differ  in  this  respect.  In  the  end, 
the  setting  should  be  checked  by  the  taking  of  indicator 
diagrams,  so  that  effects  of  incorrectible  lost  motion  may  be 
finally  eliminated. 

With  the  dead -center  points  found  the  engine  is  placed 
on,  say,  head-end  dead  center,  and  the  eccentric  shifted  until 
the  valve  is  open  to  steam  by  the  desired  lead.  The  eccen- 
tric is  then  fastened  in  this  position  and  the  engine  turned 
to  the  opposite  dead  center.  Because  of  angularity  of  con- 
nections and  of  irregularities  in  valve  and  seat  dimensions, 
it  generally  will  be  discovered  that  the  valve  is  not  now 
open  to  steam  by  the  same  amount  as  at  the  other  end. 
If  it  is  desired  that  it  should  be,  the  valve  can  be  shifted 
on  its  stem  about  half  of  the  distance  by  which  it  is  out 
and  the  eccentric  can  then  be  swung  about  the  shaft  to  take 
up  the  remaining  distance.  The  effect  should  then  be 
checked  by  putting  the  engine  on  the  opposite  dead  center. 

Valves  may  be  set  for  equal  leads  as  above,  or  for  equal 
cut-offs  or  for  any  sort  of  a  compromise  desired.  In  any 
case  the  procedure  is  about  the  same.  The  length  of  the 
valve  stem  is  adjusted,  then  the  eccentric  position  is 
adjusted,  and  then  refinements  are  effected  by  small  changes 
of  both  adjustments.  Remember  always,  that  changing 
the  length  of  the  valve  stem  changes  events  at  opposite 
cylinder  ends  in  opposite  directions,  while  shifting  the 
eccentric  changes  all  events  in  the  same  direction. 

The  effects  of  various  adjustments  are  shown  by  the 


THE  D-SLIDE  VALVE  195 

indicator  diagrams  given  in  Fig.  128.  These  diagrams  were 
taken  from  a  small,  slide-valve  engine  and  serve  very  well 
to  show  the  way  in  which  the  indicator  discloses  poor 
adjustments. 

PROBLEMS 

1.  Given:    angle  of  advance,    30°;    throw   of    eccentric,    1£ 
ins.;    lead,  ^  in.;    maximum   exhaust-port  opening,  If  in.;    find 
the  steam  lap,  maximum  opening  to  live  steam,  and  the  exhaust 
lap. 

2.  Given:    steam  lap  of  f  in.;    lead  of  ^  in.;    exhaust  lap  of 
f  in.;    and  the  angle  of  advance  equal  to  30°.     Find  the  valve 
travel  (=2X throw  of  eccentric)  and  maximum  port  opening  to 
steam  and  to  exhaust. 

3.  An  engine   has  an  eccentric  throw  of  If  ins.;    a  steam  lap 
of  f  in.;    and  a  lead  of  j^  in.     Compression  begins  at  J  of  the 
return  stroke..  Assume  a  connecting  rod  of  infinite  length  and 
find  the  angle  of  advance,  the  exhaust  lap,  and  the  maximum 
port  openings  to  steam  and  to  exhaust. 

4.  Given:   valve  travel,  3  ins.;   steam  lap,  f  in.;   exhaust  lap, 
1  in. ;  and  lead,  |  in. ;  find  maximum  port  opening,  angle  of  advance, 
and  piston  positions  at  cut-off,  release,  compression,  and  admission 
for  both  ends  of  cylinder,  with  the  length  of  the  connecting  rod 
equal  to  4£  times  the  length  of  the  crank. 

5.  It  is  required  to  build  an  engine  having  a  steam-port  opening 
of  f  in.,  a  lead  of  ^  in.,  and  a  connecting  rod  four  times  the  length 
of  the  crank.     Cut-off  must  occur  at  f  stroke  and  release  at  95% 
of  the  stroke.     Find  the  inside  and  outside  lap,  the  throw  of  the 
eccentric  and  the  fraction  of  stroke  completed  by  the  beginning 
of  compression, 


CHAPTER  XI 
CORLISS    AND    OTHER    HIGH-EFFICIENCY    ENGINES 

90.  The  Trip-cut-off  Corliss  Engine.  The  slide  valve 
has  certain  limitations  which  can  be  partly,  but  never 
wholly,  overcome.  In  most  slide-valve  gears,  for  instance, 
the  various  events  occur  more  slowly  than  is  desirable, 

and  this  is  particularly 
true  of  cut-off.  Ideal 
valves  would  open  sud- 
denly to  full  opening 
when  necessary  and 
would  close  as  suddenly 
at  the  proper  time,  and 
such  action  would  give 
minimum  throttling  loss 
and  rounding  of  corners 
of  the  diagram.  Engines 

fitted  with  such  ideal  valves  would  therefore  give  indicator 
diagrams  with  maximum  work  area  as  shown  by  the 
dotted  lines  in  Fig.  129,  the  full  lines  indicating  the  type 
of  diagram  obtained  with  the  ordinary  slide  valve. 

Again,  the  simpler  forms  of  slide  valve  involve  the  use 
of  long  ports  connecting  with  the  clearance  space  within 
the  cylinder,  thus  adding  greatly  to  the  clearance  surface 
exposed  and  to  the  cylinder  condensation.  These  ports 
serve  for  both  admission  and  exhaust,  and  their  walls  are 
therefore  periodically  cooled  by  the  exhaust  steam  with  the 
result  that  excessive  condensation  occurs  during  admission. 
Many  attempts  have  been  made  to  devise  valve  gears 
which  should  not  be  subject  to  the  limitations  of  the 

196 


FIG.  129. 


HIGH-EFFICIENCY  ENGINES  197 

simple  slide  valve.  Some  of  these  have  resulted  in  the 
development  of  the  more  complicated  slide  valves  de- 
scribed in  the  last  chapter,  but  such  designs  generally  leave 
much  to  be  desired.  One  of  the  earliest  and  most  success- 
ful solutions  was  made  by  Corliss,  who  developed  what  is 
known  as  the  trip-cut-off  Corliss  gear. 

The  long  combined  steam  and  exhaust  ports  are  elimi- 
nated by  the  use  of  four  valves,  two  for  steam  and  two  for 
exhaust.  These  are  rocking  valves  and  are  located  top  and 
bottom,  at  the  extreme  ends  of  the  cylinder,  with  their 
longitudinal  axes  perpendicular  to  those  of  the  cylinder, 
as  shown  in  Figs.  50,  51  and  52.  The  exhaust  valves  are 
located  below  so  as  to  drain  out  water  of  condensation. 
Details  of  valves  of  this  type  are  shown  in  Fig.  130. 

These  valves  may  each  be  regarded  as  an  elementary 
slide  valve  which  has  a  cylindrical  instead  of  a  flat  face, 
and  which  is  oscillated  about  a  center  near  the  face  instead 
of  being  reciprocated,  i.e.,  oscillated  about  a  center  at  an 
infinite  distance. 

The  valves  are  operated  as  shown  in  Fig.  131  by  short 
links  from  a  wrist-plate  pivoted  on  the  side  of  the  cylinder 
and  rocked  back  and  forth  about  its  center  by  means  of  an 
eccentric  operating  through  the  linkage  indicated.  The 
locations  of  the  various  pins  and  the  lengths  of  the  various 
links  are  so  chosen  that  the  valves  travel  at  high  velocity 
when  opening  and  closing,  that  they  open  very  wide,  and 
that  they  close  only  far  enough  to  prevent  leakage  and  then 
remain  practically  stationary  until  about  to  open  again. 
Throttling  losses  are  thus  decreased  and  wear  caused  by 
useless  motion  after  closure  is  minimized. 

The  opening  of  the  admission  valves  in  this  gear  is 
effected  positively  by  the  linkage  already  explained,  but 
they  are  closed  differently.  For  opening,  the  steam  link 
rotates  the  bell  crank  B  in  Fig.  132  and  thus  raises  the 
latch  C.  The  hook  on  the  end  of  one  of  the  arms  of  this 
latch  engages  the  steam  arm  which  is  fastened  on  the  end 


198 


STEAM  POWER 


HIGH  EFFICIENCY  ENGINES 


199 


200 


STEAM  POWER 


of  a  rod  which  is  slotted  into  the  end  of  the  valve.  The 
valve  is  thus  drawn  further  open  as  the  wrist  plate  revolves, 
until  the  tripping  end  D  of  the  latch  strikes  the  cam  indicated 
by  E.  This  throws  the  hook  out  of  engagement  and  thus 
disconnects  the  valve  from  the  driving  mechanism.  The 


Safety  Cam 
o     Ste'amArm, 


FIG.  132. — Brails  of  Corliss  Trip-Cut-off  Gear. 


valve  is  closed  by  tlie'action  of  a  dash  pot,  one  form  of  which 
is  shown  in  Fig.  131.  As  the  steam  arm  rises  during  the 
opening  of  the  valve  it  draws  up  the  plunger  or  piston  of  the 
dash  pot,  leaving  a  partial  vacuum  beneath  it,  and,  when  the 


20"x  48"neavy  Duty  Corliss 

110  Lb.  Steam 

6i)R.P.M. 


FIG.  133. 

valve  is  released  by  unhooking  of  the  latch,  atmospheric 
pressure  drives  the  rjlunger  down  and  thus  causes  cut-off 
to  occur..  The  action  of  a  dash  pot  is  found  to  be  unsatis- 
factory when  the  speed  of  the  engine  exceeds  about  125 
R.P.M.  and  most  Corliss  engines  with  trip-cut-off  operate 


HIGH-EFFICIENCY  ENGINES  201 

at  still  lower  speeds.  Under  such  circumstances  the  cut- 
off is  very  rapid  as  compared  with  the  piston  speed,  and  the 
diagram  shows  a  comparatively  sharp  corner  at  this  point. 
A  set  of  diagrams  obtained  from  a  large  Corliss  engine 
operating  at  80  R.P.M.  is  given  in  Fig.  133,  and  it  is  obvious 
that  little  throttling  occurs. 

Because  of  the  low  speed  at  which  these  engines  operate 
the  stroke  can  be  made  long  with  respect  to  the  diameter 
without  attaining  a  prohibitive  piston  speed.  The  economy 
mentioned  in  Chapter  VII  as  resulting  from  the  use  of  long 
strokes  can  thus  be  obtained  in  these  engines.  An  idea 
of  the  saving  in  steam  effected  by  the  partial  elimination 
of  throttling  and  condensation  losses  by  means  of  the 
Corliss  gear  can  be  obtained  from  the  curves  in  Fig.  134 
(a)  and  (6),  which  give  average  performances. 

The  position  of  the  cam  which  determines  the  time 
at  which  cut-off  occurs  is  controlled  by  the  governor  of 
the  engine.  When  moved  in  the  direction  taken  by  the 
steam  arm  it  causes  cut-off  to  occur  later.  Variation  of 
the  point  of  cut-off  is  used  in  these  and  in  most  other  engines 
to  control  the  amount  of  work  done  per  cycle  in  order  that 
the  engine  may  make  available  the  quantity  demanded  at 
the  shaft,  as  will  be  explained  in  a  later  chapter.  It  is  there- 
fore desirable  that  the  range  of  cut-off  should  be  as  great 
as  possible,  but  it  has  been  found  very  difficult  to  design 
trip-cut-off  gears  which  will  give  a  cut-off  later  than  about 
0.4  stroke  if  steam  and  exhaust  valves  are  operated  from  the 
same  eccentric.  Later  cut-off  causes  poor  timing  of  the 
exhaust  events. 

This  has  led  to  the  introduction  of  Corliss  engines 
with  two  eccentrics  and  two  wrist  plates  per  cylinder. 
One  set  operates  the  steam  valves  and  the  other  the  exhaust 
valveSi  With  this  arrangement  the  range  of  cut-off  is 
unlimited. 

91.  Non-detaching  Corliss  Gears.  Because  of  the  low 
speed  at  which  trip-cut-off  Corliss  engines  are  Qperated,, 


202 


STEAM  POWER 


HIGH-EFFICIENCY  ENGINES 


203 


sqi-uopdransuoo  tuuajg 


204 


STEAM  POWER 


they  are  necessarily  large,  heavy  and  costly  and  efforts 
have  been  made  to  design  gears  which  shall  possess  the 
advantages  of  the  original  Corliss  mechanism  without 
the  limitation  as  to  speed. 

In  many  models  the  Corliss  valves  are  retained  and  are 
located  in  the  end^s  of  the  cylinder  as  just  described  or  in 


Double-Ported 

Exhaust  Valve. 

Corliss  Type 


FIG.  135. — Non-detaching  Corliss  Valves  Located  in  Cylinder  Head. 


the  cylinder  heads  as  shown  in  Fig.  135.  In  some  the  wrist 
plate  and  the  connecting  links  are  also  retained,  but  in 
others  they  are  eliminated.  In  all  engines  of  this  type  the 
admission  valves  are  closed  positively,  the  closure  being 
effected  by  the  same  linkage  that  opens  the  valves  to  admit 
steam.  Quick  action  is  obtained  by  the  arrangement  of 
the  operating  mechanisms,  the  centers  of  rotation  and  the 


HIGH-EFFICIENCY  ENGINES  205 

lengths  of  links  being  so  chosen  that  the  valve  travel  is 
small  when  the  valves  are  closed,  that  it  is  rapid  when  the 
valves  are  opening  and  closing,  and  that  the  valves  remain 
practically  wide  open  during  most  of  the  time  that  steam 
is  being  admitted. 

The  advantages  of  small  clearance  and  short  and  sepa- 
rate ports  are  attained  in  these  arrangements  and  the 
operation  of  the  valves  is  almost  as  perfect  as  that  of  the 
trip-cut-off  gear.  Engines  fitted  with  these  modified 
Corliss  gears  are  operated  at  speeds  considerably  higher 
than  those  permissible  with  the  older  arrangement,  and  they 
may  be  classed  with  medium-speed  engines. 

Engines  of  this  type  are  generally  known  commercially 
as  four- valve  engines,  but  as  this  name  applies  equally  well 
to  the  ordinary  trip-cut-off  gear  and  to  others  which  will 
be  described  later,  it  is  best  to  use  some  other  designation. 
The  term  non-detaching  Corliss  engines  seems  to  best 
describe  them  and  is  apparently  gaining  in  favor. 

Non-detaching  Corliss  engines  generally  give  diagrams 
intermediate  between  those  obtained  with  the  low-speed, 
trip-cut-off  mechanism  and  those  obtained  from  slide-valve 
engines  with  the  simpler  forms  of  valves,  though  the  later 
designs  very  closely  approximate  the  performances  of  the 
trip-cut-off  Corliss  engine. 

92.  Poppet  Valves.  Attention  has  already  been  called 
to  the  fact  that  the  use  of  highly  superheated  steam  is 
very  effective  in  lessening  or  even  eliminating  initial  con- 
densation. Experience  has  shown  that  large  valves  and 
valves  with  sliding  surfaces  such  as  slide  valves  and  Corliss 
valves  do  not  work  well  with  highly  superheated  steam. 
The  large  castings  warp  so  that  contact  surfaces  do  not 
remain  true  and  the  lack  of  moisture  which  acts  as  a  seal 
with  saturated  steam  leads  to  excessive  leakage.  Dif- 
ficulty has  also  been  experienced  with  the  lubrication  of 
these  sliding  types  of  valves  when  using  highly  superheated 
steam. 


STEAM  POWEK 


An  old  form  of 
valve  known  as  the 
poppet  valve  has  re- 
cently been  adopted 
by  some  builders  as 
a  solution  of  the 
difficulties  met  in 
the  use  of  highly 
superheated  steam. 
This  form  of  valve  in 
four-valve  arrange- 
ment, combined  with 
designs  in  which  short 
ports  and  symmet- 
rical cylinder  cast- 
ings are  used,  yields 
very  economical  en- 
gines which  can  be 
safely  used  with  a 
degree  of  superheat 
prohibitively  high  in 
the  case  of  the  slid- 
ing and  oscillating 
forms  of  valves. 


Receiver 


FIG,  1366. — Cross-section,  Lentz 
Engine. 


HIGH-EFFICIENCY  ENGINES 


207 


Sections  of  a  modern  type  of  poppet  valve  engine  are 
shown  in  Figs.  136  (a)  and  136  (6),  and  details  of  the 
admission  valve  and  its  operating  mechanism  are  given  in 
Fig.  137  (a)  and  (6).  The  valves  are  all  double-seated 
(double-ported  or  double-beat),  that  is,  they  seat  at  both 
ends  and  are  made  hollow  so  that  the  steam  passes  both 
around  the  outside  of  the  valve  and  through  the  valve 
as  shown  by  the  arrows  in  Fig.  137  (6).  This  results  in 
large  area  for  passage  of  steam  and  in  quick  opening  and 


To  Cylindur 


FIG.  137a.— Admission  Valve  and  Operating 
Mechanism,  Lentz  Engine. 


FIG.  1376. 


closing,  as  in  the  case  of  gridiron  valves,  with  small  actual 
movement  of  the  valve. 

The  valves  are  opened  positively  by  eccentrics  opera- 
ting through  cams  and  rollers  as  shown  in  Fig.  136  (b)  and 
they  are  closed  by  springs  as  rapidly  as  the  return  motion 
of  the  cam  permits.  The  eccentrics  are  mounted  on  a 
horizontal  lay  shaft  which  is  located  to  one  side  of  the 
engine,  with  its  axis  parallel  to  that  of  the  latter,  and  which 
is  driven  by  bevel  gears  from  the  crank  shaft  of  the  engine. 

Since  this  valve  arrangement  gives  short  steam  and 
exhaust  poits,  permits  the  use  of  small  clearance,  and 


208  STEAM  POWER 

gives  fairly  rapid  opening  and  closing  of  valves  with  little 
throttling  when  open,  it  gives  good  economy  when  used 
with  saturated  steam.  By  adding  superheat  the  economy 
is  still  further  improved.  The  water  rate  of  one  of  these 
engines  is  shown  for  one  load  in  Fig.  134  (a).  A  simple, 
Lentz  non-condensing  engine  is  reported  to  have  given 
a  consumption  of  16.13  Ibs.  of  steam  per  horse-power  hour 
with  92.7°  superheat,  and  a  pressure  of  133  Ibs.,  and  this 
figure  is  materially  lowered  by  compounding,  higher  super- 
heat, lower  back  pressure,  etc. 

93.  The  Una-flow  Engine.  A  very  interesting  modifica- 
tion of  the  steam  engine,  known  as  the  Una-flow  Engine, 
has  recently  appeared.  In  this  design  an  attempt  is  made 
to  decrease  the  loss  due  to  condenastion  in  a  very  original 
way  and  the  results  of  tests  seem  to  indicate  that  the  design 
makes  possible  very  great  economy. 

In  the  ordinary  forms  of  engine  the  entire  wall  of  the 
cylinder  is  subjected  to  the  cooling  action  of  the  lowest 
temperature  steam  during  the  entire  exhaust  stroke,  and  in 
double-acting  types  these  cooled  walls  are  immediately 
brought  into  contact  with  the  higher  pressure  steam  acting 
on  the  other  side  of  the  piston,  as  well  as  coming  into  con- 
tact later  with  the  next  charge  of  high-pressure  steam. 
The  una-flow  design  minimizes  this  action  by  admitting 
steam  at  the  ends  of  the  cylinder,  exhausting  it  at  the 
center  of  length  of  the  cylinder,  and  compressing  the  steam 
caught  in  the  clearance  up  to  a  value  approximating  initial 
pressure,  thus  heating  the  clearance  walls.  The  heating 
of  the  clearance  walls  is  further  effected  by  partly  jacket- 
ing the  head  with  live  steam  on  its  way  to  the  admission 
valve  and  the  jacket  is  sometimes  extended  along  the 
cylinder  to  the  point  at  which  cut-off  normally  occurs. 

One  form  of  this  engine  is  shown  in  Figs.  138  and  139. 
The  steam  enters  the  cylinder  head  from  below,  passes 
up  to  a  double-seated  poppet  valve,  flows  into  the  cylinder 
until  cut-off  occurs  and  then  expands  until  the  piston 


HIGH-EFFICIENCY  ENGINES 


209 


uncovers  the  exhaust  ports.  The  steam  is  exhausted 
until  the  returning  piston  again  covers  these  ports,  after 
which  the  material  trapped  within  the  cylinder  is  compressed 
as  indicated  in  the  diagrams.  The  ideal  sought  is  to  main- 
tain each  part  of  the  wall  approximately  at  the  tempera- 
ture which  the  expanding  steam  will  have  when  reaching 
it  and  thus  to  minimize  thermal  interchanges  and  loss. 


FIG.  138. — Section  of  Una-flow  Engine  Cylinder. 

Tests  of  these  engines  show  that  very  great  ratios 
of  expansion  can  be  used  in  a  single  cylinder  without  the 
excessive  losses  customary  when  such  ratios  are  attempted 
in  the  ordinary  counter-flow  type.  It  is  thus  possible  to 
obtain  good  economy  with  one  una-flow  cylinder  expanding 
from  a  high  pressure  to  a  vacuum ;  conditions  which  would 
involve  the  use  of  compounding  with  ordinary  construc- 
tion. 


210 


STEAM  POWER 


The  results  of  tests  on  one  of  the  first  Una-flow  engines 
built  in  this  country  are  shown  in  Fig.  134  (a)  and  (6).  In 
comparing  with  the  curves  it  should  be  noted  that  two  of 
the  tests  were  run  with  high  superheat. 

94.  The  Locomobile  Type.  In  the  effort  to  improve 
the  ecomony  of  small  steam  plants  the  Germans  developed 
a  form  of  plant  now  known  as  the  Locomobile  Type.  The 


FIG.  139. 


name  came  from  the  fact  that  these  plants,  as  originally 
made,  were  mounted  on  wheels  and  intended  for  portable 
use  by  agriculturists  and  contractors.  Their  economy  in 
the  use  of  fuel  proved  so  great  that  they  have  since  been 
built  for  stationary  use  in  sizes  running  well  toward  1000 
horse-power  per  unit. 

A  locomobile  of  American  construction  known  as  the 
Buckeye-mobile  is  illustrated  in  Fig.  140,  which  shows  a 
longitudinal  section  of  the  plant.  The  tandem  compound 


HIGH-EFFICIENCY  ENGINES 


211 


212  STEAM  TOWER 

engine  is  mounted  on  top  of  an  internally  fired  boiler  with 
the  engine  cylinders  located  in  the  flues  which  lead  the 
products  of  combustion  away  from  the  boiler. 

The  steam  generated  in  the  boiler  is  passed  through 
a  superheater  suspended  in  the  smoke  box.  The  flow  of 
steam  is  from  the  rear  toward  the  front  of  this  superheater 
(counter  flow)  so  that  the  hottest  steam  comes  in  contact 
with  the  hottest  gas.  The  steam  then  passes  through  a 
pipe  contained  within  the  flue  to  the  high-pressure  cylinder, 
which  is  jacketed  by  the  hot  flue  gases  and  in  which  the 
loss  of  heat  to  metal  is  thus  minimized.  From  the  high- 
pressure  cylinder  the  steam  passes  to  a  receiver  contained 
in  the  smoke  box,  the  receiver  serving  as  a  reheater  to 
evaporate  any  condensate  exhausted  from  the  first  cylinder 
and  to  superheat  the  steam  admitted  to  the  low-pressure 
cylinder.  From  the  low-pressure  cylinder,  the  steam 
passes  through  a  feed-water  heater  in  which  it  raises  the  tem- 
perature of  the  boiler  feed  and  then  it  passes  to  atmosphere 
or  to  a  condenser.  Boiler-feed  pump  and  condenser  pump, 
if  used,  are  also  integral  parts  of  the  plant,  being  driven 
directly  from  the  main  engine. 

It  will  be  observed  that  every  precaution  is  taken  to 
guard  against  initial  condensation,  and  to  minimize  loss 
of  heat  in  flue  gases  and  in  exhaust  steam  leaving  the 
plant.  The  high  economies  achieved  are  due  to  such 
facts  alone. 

Small  plants  of  this  type  have  given  an  indicated  horse- 
power hour  on  a  little  over  one  pound  of  coal  when  oper- 
ated condensing,  whereas  the  best  large  compound  recip- 
rocating engine  plants  seldom  do  better  than  about  1.75 
Ibs.  of  coal  per  I.h.p.  and  often  use  2  or  more  pounds  when 
operated  condensing, 


CHAPTER  XII 
REGULATION 

95.  Kinds  of  Regulation.  There  are  two  distinctively 
different  kinds  of  regulation  referred  to  in  connection 
with  reciprocating  steam  engines,  one  of  which  may  be 
called  fly-wheel-regulation  and  the  other  governor-regula- 
tion or  governing. 

The  regulating  effect  of  the  fly-wheel  has  already  been 
referred  to.  The  turning  effort  exerted  at  the  crank  pin 
by  the  action  of  steam  on  the  piston  or  pistons  of  an  engine 
is  not  constant,  and  the  angular  velocity  of  the  engine  shaft 
is  therefore  constantly  varying  during  each  revolution. 
It  is  the  function  of  the  fly-wheel  to  damp  these  variations 
so  that  they  do  not  exceed  the  allowable  maximum  for 
any  given  set  of  operating  conditions.  The  efficiency  of 
the  fly-wheel  in  this  respect  is  measured  by  the  coefficient 
of  fly-wheel  regulation  dw  which  is  defined  by  the  equation 

-  ir*  j      -.1    •     I  .  ,  T T  T T 

j  V  max        '  min  /«A\ 

$W=         ~^T~  ~~>          ...•;.        (66) 

in  which 

T7max  =  maximum   velocity   attained   by   a   point   on 

fly-wheel  rim  or  other  revolving  part; 
V mm  =  minimum  velocity  of  the  same  point,  and 
F  =  mean  velocity  of  the  same  point 

'  max  ~i     '  rnln  •          ,    i 

approximately. 

z 

Governor-regulation  is  absolutely  different.  Its  function 
is  to  proportion  the  power  made  available  to  the  instan- 
taneous demand.  The  fly-wheel  takes  care  of  variations 

213 


214 


STEAM  POWER 


occurring  during  the  progress  of  one  cycle,  while  governor 
regulation  varies  the  work  value  of  successive  cycles. 

96.  Governor  Regulation.  If  the  effect  of  engine 
friction  be  neglected,  the  power  delivered  at  the  shaft  of 
the  engine  will  vary  directly  with  the  indicated  horse- 
power. Such  an  assumption  is  accurate  enough  for  the 
discussion  which  follows. 

The  indicated  horse-power  of  a  given  engine  is  deter- 
mined entirely  by  the  value  of  the  mean  effective  pressure 
and  the  number  of  cycles  produced  in  a  given  time,  since 
these  are  the  only  variables  in  the  formula  for  indicated 
horse-power.  The  power  made  available  by  an  engine 
might  therefore  be  varied  by  varying  the  mean  effective 


FIG.  141. — Throttling 
Governing. 


FIG.  142.— Cut-off 
Governing. 


pressure,  or  by  varying  the  number  of  cycles  produced  in 
a  given  time,  or  by  a  combination  of  both  processes. 

All  of  these  possibilities  are  used.  In  ordinary  station- 
ary power  plants  the  mean  effective  pressure  is  generally 
varied.  In  the  case  of  pumping  engines,  working  against 
a  constant  head,  but  required  to  deliver  different  quantities 
of  water  at  different  times,  the  number  of  cycles  per  minute 
is  generally  altered  by  changing  the  speed  at  which  the 
engine  operates.  In  locomotive  and  hoisting  practice 
both  the  number  of  cycles  per  minute  (speed)  and  the 
mean  effective  pressure  are  varied  as  required  to  meet 
the  instantaneous  demands. 

These  variations  may  be  effected  manually  as  by  the 
driver  of  a  locomotive,  in  which  case  the  engine  may  be 
said  to  be  manually  governed.  Or,  they  may  be  brought 


REGULATION  215 

about  mechanically,  as  in  the  case  of  most  stationary  power- 
plant  engines,  in  which  case  the  engine  may  be  said  to  be 
mechanically  governed.  In  some  instances  a  combination 
of  manual  and  mechanical  governing  is  used. 

97.  Methods  of  Varying  Mean  Effective  Pressure.     The 
mean   effective  pressure  increases  and  decreases  with  the 
area  of  an  indicator   diagram    of  constant  length,  so  that 
the  mean  effective  pressure  can  be  changed  by  any  method 
which  will  change  the  area  of  the  diagram.     Two  methods 
are  in  use  and  they  are  illustrated  in  Figs.  141  and  142. 
The  first  causes  a  variation  in  area  by  changing  the  value 
of  the  initial  pressure.     This  is  generally  done  by  chang- 
ing the  opening  of  a  valve  in  the  steam  line  just  outside  of 
the  steam  chest.     It  is  called  throttling  governing,  and  the 
valve  is  called  a  throttling  or  throttle    valve.     The  latter 
name  is  also  commonly  used  for  the  valve  located  near  the 
engine,  which  is  used  to  shut  off  the  supply  of  steam  entirely 
when  the  engine  is  not  in  operation. 

The  second  method,  illustrated  in  Fig.  142,  is  known 
as  cut-off  governing.  The  variation  of  cut-off  determines 
the  amount  of  steam  admitted  to  the  cylinder  per  cycle 
and  is  used  to  measure  out  the  quantity  required  for  the  load 
which  happens  to  exist  at  any  instant.  Cut-off  governing 
is  used  on  most  modern  stationary  engines  and  is  exclusively 
used  in  large  reciprocating  engine  power  plants. 

98.  Constant  Speed  Governing.     Most  engines  used  for 
such  purposes  as  the  operation  of  mills  and  the  driving  of 
electrical  and   centrifugal  machinery  are  required  to  run 
at  practically  constant  speed  irrespective  of  the  load.     They 
are  furnished  with  mechanical  governors  which  so  regulate 
the  power  made  available  that  there  shall  never  be  any 
appreciable  excess  or  deficiency  which  would  respectively 
cause  an  increase  or  a  decrease  in  speed. 

These  mechanical  devices  always  contain  some  sort 
of  tachometer  which  moves  whenever  the  speed  of  the  engine 
exceeds  or  falls  below  the  proper  value.  The  tachometer 


216  STEAM  POWER 

is  so  connected  to  the  valve  gear  that  it  decreases  the 
power-making  ability  of  the  engine  whenever  the  speed 
starts  to  increase  and  it  increases  the  power-making  ability 
if  the  speed  drops. 

Since  the  valve  gear  must  have  a  different  position 
for  each  load  in  order  that  it  may  throttle  or  cut  off  as 
necessary  to  suit  that  load,  it  follows  that  the  tachometer 
which  controls  the  position  of  the  valve  gear  must  also 
have  different  positions  for  different  loads.  But  tachom- 
eters assume  positions  dependent  on  speed,  and  therefore 
different  loads  can  only  be  obtained  if  the  tachometer  and 
the  engine  to  which  it  is  connected  operate  at  different 
speeds  for  different  loads. 

Constant-speed  governing  is  therefore  an  anomaly. 
The  device  which  is  supposed  to  maintain  constant  speed 
irrespective  of  load  must  be  operated  at  different  speeds, 
as  the  load  varies,  in  order  that  it  may  maintain  the  valve 
gear  in  the  different  positions  required  to  handle  the  differ- 
ent loads.  All  so-called  constant-speed  engines  have  their 
highest  speed  when  carrying  no  load,  and  the  speed  gradually 
decreases  to  a  minimum  as  the  load  increases  to  a  maxi- 
mum. The  total  variation  is  generally  between  2  and  4%. 

The  efficiency  of  a  governor  in  this  respect  is  measured 
by  means  of  the  coefficient  of  governor  regulation,  do, 
which  is  defined  by  the  equation 


in  which 

n2  =  highest  rotative  speed  attained  by  the  engine; 
m  =  lowest  rotative  speed  attained  by  the  engine,  and 
n  =  mean  speed 

ri2+n\ 
=  —  ;r  —  approximately. 

99.  Governors.     The    mechanical    devices    which    are 
used  for  controlling  the  power-making  ability  of  an  engine 


REGULATION 


217 


as  described  above  are  known  as  governors.  There  are 
many  varieties  and  only  a  few  of  the  more  prominent  can 
be  described. 

(a)  The  Pendulum  Governor.  One  of  the  earliest  forms 
of  governor  used  on  steam  engines  is  illustrated  in  Fig. 
143.  It  is  often  called  a 
fly-ball  governor.  This 
governor  is  driven  by 
gearing,  chain  or  belt 
from  the  engine,  and  the 
weights  assume  some 
definite  position  for 
each  different  speed,  thus 
drawing  the  collar  to 
different  positions.  The 
valve  gear  is  connected 
to  this  collar  and  is 
moved  correspondingly. 

A  similar  governor 
is  shown  in  Fig.  131, 
which  also  indicates  the 

way  in  which  the  collar  is  connected  to  the  valve  gear  in 
the  Corliss  type  of  engine.  The  governor  rods  are  moved 
as  the  collar  moves  and  they  in  turn  alter  the  position 
of  the  knock-off  cam,  and  thus  vary  the  time  at  which 
cut-off  occurs.  As  the  speed  increases  due  to  a  decrease 
of  load,  the  governor  weights  and  collar  move  up,  and  this 
shifts  the  cams  so  as  to  produce  earlier  cut-off  and  decrease 
power-making  ability. 

(6)  Shaft  Governors.  On  medium-  and  high-speed 
engines  fitted  with  some  form  of  slide  valve  it  is  found  best 
to  use  what  are  known  as  shaft  governors.  They  are  gen- 
erally carried  within  the  fly-wheel  of  the  engine,  operate  in 
a  plane  passing  through  the  rim  of  the  wheel  at  right  angles 
to  the  shaft,  and  operate  upon  the  eccentric  in  such  a  way 
as  to  vary  the  cut-off  with  speed  (and  load)  changes. 


FIG.  143. 


218 


STEAM  POWER 


FIG.  144. 


One  simple  form  of  such  a  governor  is  shown  in  Fig. 
144.     The    eccentric    is    not    mounted    directly    upon    the 

engine  shaft,  but  is  carried 
by  a  pin  P  in  the  fly-wheel 
and  is  slotted  so  that  it  can 
swing  back  and  forth  across 
the  shaft,  about  P  as  a  center. 
Its  position  at  any  time  is 
determined  by  the  position 
of  the  governor  weights  W, 
which  draw  the  eccentric 
down  (in  the  figure)  as  they 
move  out. 

The  center  of  the  eccentric 
is  indicated  by  a  heavy  dot 
in  the  figure,  and  it  will  be  seen  that  this  center  would 
travel  in  the  arc  of  a  circle  about  P,  as  the  weights  moved. 
If  the  path  of  the  eccentric  center  is  drawn  on  a  Bilgram 
diagram,  it  will  be  found  that  this  motion  is  equivalent  to 
decreasing  the  length  of  the  eccentric  crank  and  increasing 
the  angle  of  advance,  resulting  in  earlier  cut-off  as  the 
weights  move  out  with  increasing  speed  and  decreasing 
load.  Other  events  will  also  be  changed  as  the  eccentric 
swings,  and  some  of  these  changes  are  occasionally  unde- 
sirable. 

Numerous  designs  have  been  developed  in  which  the 
eccentric  is  so  guided  as  to  produce  various  sorts  of  rela- 
tions between  the  different  steam  and  exhaust  events. 
All  can  be  divided  into  two  classes,  those  in  which  the 
eccentric  swings  about  a  fixed  center  variously  located,  and 
those  in  which  the  center  of  the  eccentric  is  guided  to 
move  in  a  straight  line.  All  can  be  studied  by  plotting 
the  path  of  the  eccentric  center  (path  of  Q)  on  the  Bilgram 
diagram. 

The  Rites  Inertia  Governor  is  a  form  of  shaft  governor 
so  designed  as  to  act  very  quickly  with  change  of  speed, 


REGULATION 


219 


and  to  be  very  powerful,  so  that  it  can  shift  heavy  parts. 
It  is  shown  in  place  in  the  wheel  in  Fig.  145.  With  changes  in 
speed  it  acts  like  a  governor  of  the  type  just  described, 
swinging  (with  increasing  speed)  about  a  fixed  point  P  in 
the  wheel  as  its  center  of  gravity  G  moves  outward  under 
the  action  of  the  centrifugal  effect  C  and  against  the 
action  of  the  spring.  This  motion  shifts  the  center  of  the 


FIG.  145. 

eccentric  from  E  toward  c,  giving  the  desired  variation  in 
cut-off. 

Superposed  upon  this  action  is  that  of  inertia.  Assume 
the  wheel  and  governor  to  be  rotating  clockwise  at  a  given 
constant  speed.  If  the  engine  speed  is  suddenly  increased, 
the  wheel  will  move  faster,  but  the  governor  bar  will  tend 
to  continue  rotating  at  the  same  speed  because  of  its  inertia. 
It  will  thus  lag  behind  the  wheel,  rotating  about  P  and  bring- 
ing about  an  earlier  cut-off.  The  position  thus  assumed 
will  later  be  maintained  by  centrifugal  effect  if  the  new  speed 


220  STEAM  POWER 

is  maintained.  The  particular  advantage  resulting  from 
using  inertia  in  this  way  is  speed  of  action.  In  many  forms 
of  governor  the  inertia  of  the  moving  parts  actually  resists 
the  efforts  of  the  governor  to  assume  the  new  position 
required  by  changed  load  and  speed. 


CHAPTER  XIII 
THE  STEAM  TURBINE 

100.  The  Impulse  Turbine.  One  of  the  oldest  of  modern 
water  wheels  is  the  tangential  or  impulse  wheel  shown 
diagrammatically  in  Fig.  146.  Water  flowing  from  a 
reservoir  above  the  wheel  passes  through  a  nozzle  and  the 


FIG.  146. — Tangential  or  Impulse  Wheel. 

jet,  moving  at  high  velocity,  strikes  buckets  on  the  rim 
of  the  wheel  and  causes  the  latter  to  revolve.  Theoretically 
the  velocity  of  the  water  in  the  jet  would  be 

v  =  V2gh  feet  per  second,   .     .     .     «     (68) 
in  which 

g  =  gravitational  constant,  32.2,  and 
h  =  head  in  feet  as  shown  in  the  figure. 

The  kinetic  energy  possessed  by  the  moving  water  would 
be 

*  =  f  4      111    .     .     .     (69) 
221 


222  STEAM  POWER 

in  which  w  represents  pounds  of  water  discharged  per  second 
and  g  and  v  have  the  same  meanings  as  above. 

If  the  buckets  of  the  wheel  could  reduce  the  velocity 
of  the  water  to  zero  they  would  absorb  all  of  this  kinetic 
energy  and  (assuming  no  losses  within  the  buckets  and  the 
bearings  of  the  wheel)  would  make  all  of  it  available  at 
the  shaft  for  the  doing  of  useful  work. 

Any  fluid  moving  at  velocity  v  and  striking  buckets  in 
the  form  of  a  jet  would  possess  kinetic  energy  in  quantity 
given  by  Eq.  (69)  and  would  drive  the  wheel  in  the  same 
way.  Steam  might  therefore  be  used  instead  of  water 
with  exactly  the  same  results,  and  steam  is  so  used  in  what 
are  known  as  impulse  steam  turbines. 

Experience  shows  that  steam  will  flow  at  high  velocity 

from  any  opening  made  in  the 
steam  space  of  a  boiler  or 
from  any  open-ended  pipe  con- 
nected to  such  a  boiler.  This 
is  commonly  said  to  be  due 
to  the  high  pressure  within 
the  boiler,  the  spectator  pic- 
turing  the  process  as  the 
driving  out  of  part  of  the 

steam  by  the  high-pressure  steam  within  the  boiler,  just  as 
though  the  part  leaving  were  a  solid  piston  and  were  driven 
out  as  is  the  piston  of  an  engine  during 
admission,  as  shown  in  Fig.  147,  ^ 

An  hydraulic  analogy  is  given  in  Fig. 
148.  The  vessel  shown  is  supposed  to  be 
fitted  with  a  piston,  and  it  is  assumed  to  be  possible  to  exert 
any  desired  pressure  upon  the  piston.  Any  such  pressure 
exerted  is  the  exact  equivalent  of  some  given  head  of  water 
and  the  resultant  jet  velocity  would  be  given  by  Eq.  (68) 
by  substituting  for  h  the  head  in  feet  equivalent  to  the 
pressure  exerted  upon  the  piston. 

When  an  "  elastic  "  fluid  such  as  steam  is  being  con- 


THE  STEAM  TURBINE  223 

sidered  it  is,  however,  necessary  to  take  account  of  other 
factors.  The  steam  within  the  boiler  exists  at  a  high 
pressure ;  after  issuing  it  exists  in  the  atmosphere  at  a  lower 
pressure.  But  low-pressure  steam  contains  less  heat  than 
does  steam  at  high  pressure,  and  this  difference  must  exist 
in  some  form,  as  it  is  energy  and  could  not  possibly  have  been 
destroyed  during  the  flow. 

Experiment  shows  that  steam  after  flowing  into  the 
atmosphere  from  a  boiler  in  this  way  has  exactly  the  same 
characteristics  as  though  it  had  expanded  adiabatically 
behind  a  piston  through  the  same  temperature  range,  ex- 
cepting for  the  fact  that  it  has  a  very  high  velocity,  which 
it  would  not  possess  if  expanded  behind  a  piston.  Experi- 
ment further  shows  that,  if  small  losses  be  neglected,  the 
kinetic  energy  possessed  by  a  jet  of  steam  is  exactly  equal 
to  the  energy  which  would  be  turned  into  work  if  that  steam 
acted  on  a  piston  as  in  an  ordinary  engine. 

A  complete  picture  of  the  process  of  flow  can  then 
be  made  by  assuming  the  steam  flowing  out  in  the  form  of 
a  piston  driven  by  high-pressure  steam,  as  before,  and  adding 
to  this  the  idea  that  this  piston  expands  adiabatically  as 
it  travels  from  the  region  of  high  to  that  of  low  pressure. 
This  expansion  liberates  heat  contained  within  the  piston 
or  plug  of  steam  and  this  heat  is  used  in  imparting  addi- 
tional velocity  to  the  moving  steam  which  is  giving  up  this 
heat. 

The  result  of  using  such  a  jet  upon  a  theoretically 
perfect  tangential  or  impulse  wheel  would  be  to  rob  the 
jet  of  all  this  energy.  But  the  energy  possessed  per  pound 
of  steam  in  the  jet  i^-just  the  same  as  that  shown  under  the 
upper  lines  of  a  complete  expansion  cycle  using  one  pound 
of  steam.  The  area  under  the  upper  horizontal  line  of  the 
PF-diagram  of  the  cycle  as  shown  in  Fig.  21  may  be  assumed 
to  represent  the  work  done  upon  one  pound  of  steam  (flow- 
ing out)  by  another  pound  which  is  being  evaporated  and 
pushing  out  the  first  in  order  to  make  room  for  itself.  The 


224 


STEAM  POWER 


area  under  the  expansion  curve  in  the  PF-diagram  repre- 
sents the  energy  converted  into  velocity  energy  by  the  adia- 
batic  expansion  of  the  flowing  steam.  The  lower  hori- 
zontal line  represents  the  negative  work  during  condensa- 
tion to  water  at  the  lowest  pressure  and  temperature,  and 
the  left-hand  line  represents  the  pumping  of  this  water  back 
into  the  boiler  and  the  raising  of  its  temperature  to  the  value 


i>ianbragm 


FIG.  149. — Early  Form  of  Impulse  Turbine. 

maintained  within  the  boiler.  The  complete  expansion  cycle 
is  therefore  the  cycle .  upon  which  the  impulse  steam  turbine 
operates  and,  as  a  matter  of  fact,  it  is  the  theoretical  cycle 
of  all  steam  turbines. 

The  ideal  impulse  turbine  would  therefore  be  acted 
upon  by  a  jet  which  possessed  available  kinetic  energy 
represented  by  the  area  of  the  complete  expansion  cycle. 
If  the  buckets  could  entirely  remove  this  energy,  that  is, 
could  reduce  the  velocity  of  the  jet  to  zero,  the  same  amount 


THE  STEAM  TURBINE 


225 


of  energy  could  theoretically  be  made  available  at  the  shaft 
of  the  turbine. 

An  example  of  a  simple  form  of  impulse  steam  turbine 
is  given  in  Fig.  149,  in  which  the  essential  parts  of  an  early 
form  of  Kerr  turbine  are  shown.  The  wheel,  the  diaphragm 
and  nozzles  are  all  inclosed  within  a  casing.  The  space 
on  one  side  of  the  diaphragm  is  connected  to  the  steam  pipe 
and  that  on  the  other  is  in  communication  with  the  space 
into  which  the  exhaust  steam  is  to  be  exhausted. 

Another  form  of  impulse  turbine  is  shown  in  Fig.  157. 
It  will  be  described  later. 

101.  Theoretical  Cycle  of  Steam  Turbine.  It  was  shown 
in  the  preceding  section  that  the  steam  turbine  operates 
on  the  complete  expansion  cycle.  If  a  turbine  could  remove 
from  the  steam  passing  through  it  and  convert  into  mechan- 
ical form  all  of  the  energy  which  is  theoretically  possible, 
it  would  therefore  make  available  mechanical  energy 
represented  by  the  area  of  the  PF-diagram  of  the  complete 
expansion  cycle.  The  area  of  the  corresponding  T<f>- 
hagram  would  show  the 
Same  quantity  measured  in 
thermal  units.  The  theory 
of  the  steam  turbine  can 
therefore  be  studied  by 
means  of  these  two  dia- 
grams. 

In  Fig.  150  is  shown  the 
T^-diagram  of  the  complete 
expansion  cycle  for  several 
different  conditions.  The 
figure  abed  represents  con- 
ditions when  the  steam  is 
dry  and  saturated  at  the  beginning  of  the  adiabatic  ex- 
pansion cd.  Constant  quality  lines  are  designated  by  x 
and  x'.  It  is  obvious  that  by  the  time  the  steam  has 
expanded  down  to  the  pressure  at  d  it  will  have  a  quality 


FIG.  150. 


226  STEAM  POWER 

less  than  unity.  If,  therefore,  it  be  in  the  form  of  a  jet 
issuing  from  a  nozzle  and  having  a  high  velocity  by  virtue 
of  its  adiabatic  expansion,  the  jet  will  really  be  a  mixture 
of  steam  and  water. 

If  the  steam  be  superheated  at  constant  pressure  as  shown 
by  ce  before  passing  through  the  nozzle,  it  is  evident  from  the 
figure  that  the  jet  issuing  from  the  nozzle  will  contain 
less  water  than  in  the  preceding  case,  because  the  condition 
of  the  material  in  the  jet  after  adiabatic  expansion  will  be 
as  shown  at  /  instead  of  as  shown  at  d.  The  cycle  in  such 
a  case  would  also  be  larger  by  an  amount  indicated  by  the 
area  cefd,  representing  just  that  much  more  heat  converted 
into  mechanical  energy  per  pound  of  steam  or  other  unit 
for  which  the  diagram  happened  to  be  drawn. 

If  superheating  had  been  carried  to  the  point  indicated 
by  g  before  expansion,  the  jet  would  obviously  issue  from 
a  nozzle  in  the  form  of  superheated  steam  as  shown  by  the 
point  h  in  the  figure.  In  that  case  the  cycle  would  be 
abcgha,  and  superheat  would  have  to  be  removed  from  the 
low-pressure  steam  to  bring  it  to  the  conditions  indicated 
at  i  before  condensation  could  begin. 

If  desired,  the  PF-diagrams  for  such  cycles  can  be  drawn 
very  easily.  The  line  be,  or  be  or  by  is  a  horizontal  line  in 
the  PF-diagram.  The  line  ha  is  similarly  horizontal  and  the 
line  ab  is  vertical.  The  adiabatic  expansion  is  represented 
by  a  curved  line  in  the  PF-diagrams,  but  can  be  drawn 
easily  because  the  necessary  data  are  obtainable  from  the 
T<£-diagram,  in  which  this  expansion  is  represented  by  a 
straight  line. 

ILLUSTRATIVE   PROBLEM 

Draw  the  PF-diagram  for  a  steam  turbine  receiving  one  pound 
of  steam  at  a  pressure  of  200  Ibs.  absolute,  with  a  tempera- 
ture of  500°  F.  and  exhausting  against  a  pressure  of  0.5  Ibs. 
absolute. 

First,  locate  on  a  TV-chart  for  steam  the  point  representing 
the  condition  of  steam  at  200  Ibs.  pressure  with  a  temperature 


THE  STEAM  TURBINE 


227 


of    500°    F.,    and    draw    a    vertical    line    extending    downward 
until   it  cuts    the  horizontal    temperature   line    corresponding  to 


3      O 


•sqyui'bg  aad  'sqi- 


0.5  Ib.  pressure.     This  is  practically  at  540°  F.  absolute,  or  about 
80°  F. 


228  STEAM  POWER 

Second,  take  from  the  steam  table  the  volumes  of  one  pound 
of  steam  at,  say,  200  Ibs.,  140  Ibs.,  and  100  Ibs.  absolute  pressure 
when  superheated  to  the  values  shown  by  this  vertical  line.  These 
will  be  about  2.75  cu.ft.,  3.58  cu.ft.,  and  4.67  cu.ft.,  respectively. 
Plot  these  volumes  with  corresponding  pressures  on  a  PF-chart 
as  shown  in  Fig.  151. 

Third,  take  from  the  !F</>-chart  the  pressures  at  which  the 
vertical  line  intersects  different  volume  lines  in  the  wet  steam 
region  and  plot  volumes  against  pressures  on  the  TV-chart. 

Fourth,  draw  a  smooth  curve,  as  shown,  through  all  points 
so  determined. 

Fifth,  draw  horizontal  top  and  bottom  lines  and  a  vertical 
line  at  the  left  of  the  diagram.  This  vertical  line  should  be  to 
the  right  of  the  pressure  axis  by  an  amount  representing  the 
volume  of  one  pound  of  water,  but  the  volume  is  so  small  that 
it  cannot  be  plotted  to  any  ordinary  scale. 

102.  Nozzle  Design.  It  was  stated  in  preceding  sec- 
tions that  the  energy  which  would  be  converted  into  work 
by  the  introduction  and  adiabatic  expansion  of  steam 
behind  a  piston  is  converted  into  kinetic  energy  when  steam 
flows  out  of  an  orifice  or  nozzle  and  that  an  ideal  impulse 
turbine  could  absorb  all  this  kinetic  energy  from  the  jet, 
bringing  it  to  rest  and  making  the  energy  available  in  the 
form  of  useful  power  at  its  shaft.  It  is,  therefore,  of  interest 
to  determine  the  velocity  which  a  jet  will  acquire  under 
different  conditions. 

This  could  be  done  by  evaluating  the  area  of  a  diagram, 
such  as  that  of  Fig.  151,  and  then  putting  this  value  in 
place  of  K  in  Eq.  (69)  and  solving  for  v,  but  it  can  be  done 
much  more  accurately  and  expeditiously  in  other  ways. 
The  heat  energy  which  can  be  converted  into  kinetic 
energy  of  the  moving  jet  and  which  can  later  be  con- 
verted into  useful  work  by  the  turbine  wheel  is  represented 
by  the  area  enclosed  within  the  lines  of  the  complete 
expansion  cycle  when  drawn  on  the  T^-diagrams.  That 
is  the  area  abed  in  Fig.  152,  for  instance,  for  the  case  of 
wet  steam  at  the  beginning  of  expansion.  But  this  area 
is  equal  to  that  representing  the  heat  supplied  minus 


THE   STEAM  TURBINE 


229 


that   representing   the   heat   rejected,    that  is,    Qi  —  (J2,    so 
that 

X(in  B.t.u.)=Qi-Q2.     .     .     .    >     (70) 

The  values  of  Qi  and  Q2  can  be  found  very  readily  by 
plotting  the  points  c  and  d  upon  a  jT</>-chart  for  steam  and 
observing  the  constant  heat  lines  upon  which  they  fall, 


FIG.  152. 

or  they  can  be  obtained  even  more  conveniently  from  what 
is  known  as  a  Mollier  Chart  for  steam.  In  this  chart, 
entropy  above  32°  F.  is  plotted  against  heat  above  32°  F. 
as  shown  in  Fig.  153.  An  adiabatic  expansion  on  this  chart 
is  shown  by  a  horizontal  line,  since  this  shows  a  constant 
entropy  change  just  as  a  vertical  line  on  the  T<j>  chart  shows 
a  constant  entropy  change. 

If  a  point  is  found  in  this  chart  giving  conditions  corre» 
spending  to  those  at  point  c  in  Fig.  152,  the  value  of  Qi 


230 


STEAM  POWER 


THE  STEAM  TURBINE  231 

can  be  read  directly  under  that  point  on  the  horizontal 
axis.  A  horizontal  line  drawn  from  that  point  to  the 
terminal-pressure  line  will  give  the  point  corresponding  to 
d  of  Fig.  152  and  the  value  of  $2  can  be  read  on  the  hori- 
zontal axis  immediately  below  that  point.  The  difference 
between  the  two  readings  gives  the  value  of  the  kinetic 
energy  K  or  of  the  mechanical  energy  which  an  ideal  tur- 
bine could  make  available,  but  the  expression  will  be  in 
British  thermal  units  and  not  in  foot-pounds. 

This  value  of  the  kinetic  energy,  i.e.,  K  =  Q\  —  Qz,  may 
then  be  placed  in  Eq.  (69),  giving, 


3)=-ft.-lbs.,       .     .     .     (71) 
since  Qi  and  $2  refer  to  one  pound  of  steam,  or 

Q2)=y-ftAbs.,    .     .     .     (72) 


when  w  represents  the  number  of  pounds  of  steam  flowing 
per  second. 

Solving*cither  Eq.  (71)  or  Eq.  (72)  for  v  gives, 

v  =  V778X2g(Qi-Q2) 


Q1-Q2  feet  per  second.    .     .     (73) 


The  design  of  a  nozzle  consists  simply  in  choosing  such 
sections  that  the  desired  amount  of  steam  may  flow  through 
it  with  the  desired  pressure  drop,  as  the  velocity  obviously 
is  determined  by  that  pressure  drop.  This  is  very  con- 
veniently done  by  working  in  terms  of  one  pound  of  steam, 
since  all  formulas  and  charts  are  generally  given  on  that 
basis,  and  then  multiplying  the  cross-sectional  areas  found 
by  the  number  of  pounds  of  steam  required. 

Assume  for  instance  that  it  is  desired  to  design  a  nozzle 


232  STEAM  POWER 

to  pass  one  pound  of  steam  per  second  with  an  initial  pres- 
sure of  100  Ibs.  per  square  inch  abs.,  and  a  terminal 
pressure  of  60  Ibs.,  the  steam  being  initially  dry  and  satu- 
rated. 

The  Mollier  chart  shows  that  Qi  is  equal  to  about  1187 
B.t.u.  per  pound  of  steam,  while  Q%  is  equal  to  about  1147 
B.t.u.  The  velocity  with  which  a  jet  would  issue  from  a 
theoretically  perfect  nozzle  under  these  conditions  may 
then  be  found  by  using  Eq.  (73).  This  gives 

v  =  224  Vl  187  -1147 
=  1416  feet  per  second. 

The  shape  of  the  entrance  end  of  the  nozzle  is  generally 
made  such  that  the  steam  will  enter  it  without  great  dis- 
turbance and  the  shape  beyond  that  point  is  determined 
by  methods  which  will  be  explained  below.  The  cross- 
section  of  the  discharge  end  must  be  such  as  to  pass  the 
required  quantity  at  the  velocity  found  above  to  be  equal 
to  1416  feet  per  second.  This  is  easily  done  by  deter- 
mining the  volume  of  steam  discharged. 

Drawing  the  adiabatic  expansion  on  the  Tc/>-chart  will 
give  the  quality  at  the  end  of  the  expansion;  or,  the  quality 
can  be  determined  by  finding  what  quality  a  pound  of 
steam  at  60  Ibs.  pressure  must  have  to  give  it  a  heat  content 
of  1147  as  found  above.  With  the  quality  known  the  ter- 
minal volume  per  pound  can  be  found  by  multiplying  the 
quality  by  the  specific  volume  at  terminal  conditions. 
Thus  for  the  case  under  discussion  the  quality  will  be 
about  96.7%  and  as  the  specific  volume  at  60  Ibs.  is  7.17 
cubic  feet,  the  volume  to  be  passed  per  second,  per  pound 
of  steam  is  0.967X7.17  =  6.94  cu. ft  approximately.  If  the 
velocity  is  1416  feet  per  second  the  area  per  pound  of  steam 
must  be  6.94 -^  1416  =  0.0049  sq.ft. 

The  exact  shape  of  the  nozzle  is  determined  by  deciding 
upon  the  way  in  which  pressure,  or  velocity,  or  volume 


THE  STEAM  TURBINE 


233 


shall  change  as  the  steam  passes  through  it.  Suppose,  for 
instance,  that  a  nozzle  is  to  be  constructed  of  the  length 
shown  by  ab  in  Fig.  154,  and  that  the  pressure  is  to  vary 
along  its  length  as  shown.  Assume  also  that  the  nozzle 
is  to  pass  10  Ibs.  of  steam  per  second.  Taking  initial 
pressure  as  100  Ibs.  and  terminal  as1  60  Ibs.,  the  conditions 


Pressure,  Lbs.  per  Sq.  In. 

§  2  g  88 

1400 
1200 
1000 
800 

400 
200 

s 

^ 







5 

.__  — 

—  • 



/• 

"' 

, 

, 

N; 

^ 

r. 

>- 

^^-" 

„  ' 

X 

/ 

^ 

7 

/ 

^ 

^--^ 

to 

riati 

an  o: 

Vel( 

>city 

T.f»ii«rt.h  r»f  "NToy/lfi 

FIG.  154. — Nozzle  Design. 

will  be  the  same  as  in  the  problem  above.     The  discharge 
area  will  have  to  be  10X0.0049  sq.ft.  or  0.049  sq.ft. 

The  area  at  the  plane  x%  must  be  that  required  to  pass 
the  steam  when  it  has  the  velocity  resulting  from  expansion 
from  100  down  to  64  Ibs.,  just  as  though  the  nozzle  ended  at 
that  point.  This  can  be  found  just  as  the  terminal  area 
was  found  above.  Similarly  the  sections  at  x\,  and  x  can 
be  found  by  figuring  velocity  and  area  for  expansions  to 


234 


STEAM  POWER 


74  and  90  Ibs.,  respectively.  If  the  various  areas  required 
are  determined  in  this  way,  the  nozzle  will  have  a  longi- 
tudinal section  about  as  shown  by  the  dotted  lines  in  the 
figure  and  the  variation  of  velocity  will  be  about  as  shown 
by  the  curve. 

If  the  shape  of  a  nozzle  is  determined  in  the  same  way 
for  a  case  in  which  the  terminal  pressure  is  less  than  about 
0.58  of  the  initial  pressure,  the  nozzle  will  be  found  to  have 

a  very  different  shape.  This  is 
shown  in  Fig.  155.  The  nozzle 
is  known  as  an  expanding  nozzle 
and  the  smallest  section  is  known 
as  the  neck.  The  pressure  Pn 
in  the  neck  is  always  equal  to 
about  0.58  PI  and  the  velocity 
in  the  neck  is  always  equal  to 
just  over  1400  feet  per  second. 
It  is  therefore  the  section  at 
the  neck  which  determines  the 
quantity  of  steam  which  a  nozzle 
will  discharge  if  expanding  to  a 
pressure  equal  to  or  lower  than 
0.58  Pi. 

103.    Action     of     Steam    on 
Expanding  Nozzle.  ^^    ^^       R    hag    ^ 

stated  that  the  steam  acting  in  an  impulse  type  of  turbine 
delivers  energy  to  the  wheel  of  the  turbine  by  giving  up 
its  kinetic  energy.  In  an  ideal  turbine  the  steam  jet  would 
be  brought  to  rest  and  would  thus  give  up  all  of  its  kinetic 
energy. 

In  real  turbines  it  is  impossible  to  bring  the  jet  to  rest, 
as  practical  design  problems  prevent  it.  There  is  there- 
fore always  a  loss  in  real  machines  because  of  the  residual 
or  terminal  velocity  of  the  steam  as  it  leaves  the  wheel. 
Thus  let  the  black  section  in  Fig.  156  represent  the  section 
of  a  bucket  or  blade  sticking  out  radially  from  the  rim  of  a 


THE  STEAM  TURBINE 


235 


wheel,  the  wheel  revolving  about  the  axis  indicated  by  the 

dot  dash  line  but  located  behind  the  plane  of  the  paper. 

If  minimum  loss  by  eddying  is  to  be  experienced  at  the 

point   at   which   the    steam  jet 

enters  the  blade,   the  jet  must 

enter  the  blade  along  a  tangent 

to  the  curve  of  the  inside  of  the 

blade  at  the  entrance  edge.     This 

direction  is  shown  by   the  line 

marked  vr  in  the  figure. 

Were  the  bucket  stationary, 
the  steam  jet  would  move  as 
shown  by  iv,  but  as  the  bucket 
moves  ahead,  and,  so  to  speak, 
runs  away  from  the  jet,  the 
steam  must  really  travel  in  a 
direction  such  as  that  indicated 
by  va  in  order  to  strike  the 
bucket  in  the  direction  indicated 
by  vr.  The  conditions  governing 
the  flow  of  steam  into  a  bucket 
are  the  same  as  those  governing  the  speed  with  which  and 
direction  in  which  an  individual  runs  toward  and  jumps 
upon  a  moving  vehicle.  He  will  experience  least  shock 
when  he  is  moving  ahead  at  the  same  rate  as  the  vehicle 
at  the  instant  when  he  gets  on  board.  His  motion  must 
therefore  be  made  up  of  two,  one  toward  the  vehicle  and 
the  other  in  the  direction  of  the  vehicle's  travel. 

In  the  case  of  steam  flowing  onto  a  blade  as  shown  in 
Fig.  156,  the  various  velocities  are  so  related  that  when 
drawn  to  scale  the  real  or  absolute  velocity  of  the  steam, 
va,  and  the  real  or  absolute  velocity  of  the  blade,  vb,  form 
two  sides  of  a  triangle  of  which  the  closing  side  represents 
vr)  the  velocity  of  the  steam  relative  to  the  bucket.  The 
value  and  direction  of  vr  is  obviously  obtained  from  va  by 
geometrically  subtracting  the  velocity  of  the  bucket. 


FIG.  156 


236  STEAM  POWER 

After  entrance,  the  steam  flows  around  the  inner  curve 
of  the  blade  and  is  finally  discharged  with  the  same  rela- 
tive velocity  as  that  with  which  it  entered,  and  at  an  angle 
set  by  the  tangent  to  the  inner  curvature  of  the  discharge 
edge  of  the  blade  as  shown  by  VR.  But,  since  the  steam 
has  been  moving  ahead  with  the  same  velocity  as  the 
bucket  during  the  entire  time  that  it  was  in  contact  with 
the  bucket,  it  is  also  moving  ahead  with  a  velocity  Vb  when 
it  leaves  the  wheel.  Its  real  or  absolute  velocity  is  then 
VA,  which  is  found  by  combining  VR  and  VD  as  shown  in  the 
figure. 

The  kinetic  energy  possessed  by  the  jet  when  entering 

*/'/'    2> 

the  blade  is  equal  to  -~-  ft.-lbs.,  and  that  which  it  possesses 

2 

when  leaving  is        *   .     Obviously,   the    energy    removed 


U'l  •  J.U         Ul      J         •  <  Tf      4-U        Ul      J 

while  passing  over  the  blade  is  -^  ---  ~  —  '•     I*   the  blade 

were  theoretically  perfect,  it  would  be  so  constructed  that 
rA2  would  be  zero  and  all  of  the  kinetic  energy  would  then 
l)e  removed.  This  is  practically  impossible  in  a  real  mechan- 
ism, and  there  is  always  a  loss  due  to  the  residual  velocity 
VA.  The  best  that  can  be  done  is  to  so  choose  the  angle 
of  jet  and  blade,  and  the  velocity  of  blade  with  respect 
to  the  steam  that  the  actual  numerical  value  of  VA  is  made 
as  small  as  possible. 

Designs  usually  work  out  in  such  a  way  that  this  occurs 
when  the  blade  velocity  is  equal  to  about  0.47  of  the  abso- 
lute velocity  of  the  steam  jet. 

104.  De  Laval  Impulse  Turbine.  The  expanding  nozzle 
already  described  was  first  used  by  De  Laval  in  an  impulse 
type  of  turbine.  The  essential  elements  of  this  device  are 
shown  in  Fig.  157.  The  nozzles  are  arranged  at  such  an 
angle  to  the  plane  of  the  wheel  that  the  steam  jets  strike 
radially  arranged  blades  at  the  proper  angle  to  enter  without 
much  loss.  The  blades  deflect  the  jets  as  shown  and 


THE  STEAM  TURBINE 


237 


absorb  the  greater  part  of  their  kinetic  energy,  so  that 
the  steam  flows  away  from  the  wheel  with  low  absolute 
velocity. 

As  many  nozzles  are  used  as  are  required  to  make  avail- 


Nozzle 


Turbine  Shaft 


Steam  In 


Nozzles  - 
FIG.  157. — Single  Stage,  De  Laval  Impulse  Turbine. 

able  the  amount  of  energy  desired  at  full  load,  and  pro- 
vision is  made  for  shutting  off  one  or  more  nozzles  by  hand 
when  conditions  do  not  warrant  the  use  of  all.  Governing 
for  ordinary  variations  of  load  is  effected  by  throttling 
the  steam  flowing  to  the  nozzles  in  use,  thus  altering  the 
initial  pressure  as  necessary. 


238  STEAM  POWER 

A  section  through  the  wheel  and  casing  of  such  a  tur- 
bine directly  connected  to  a  centrifugal  pump  is  given 
in  Fig.  158.  The  steam  flows  into  the  live  steam  space 
through  a  throttle  valve  controlled  by  the  governor;  the 
valve  and  connections  are  not  shown  in  the  illustration. 
From  the  live  steam  space  the  steam  flows  through  nozzles 
not  shown,  and  into  the  exhaust  steam  space,  thus  acquir- 
ing a  high  velocity.  The  buckets  of  the  wheel  are  located 
just  in  front  of  the  discharge  ends  of  the  nozzles  and  the 
steam  moving  at  high  velocity  must  pass  through  them 
before  moving  on  toward  the  exhaust  outlet. 

105.  Gearing  and  Staging.  It  has  been  stated  that  the 
most  efficient  operation  with  ordinary  designs  is  obtained 
when  the  blade  speed  is  equal  to  about  0.47  of  the  absolute 
steam  velocity  or,  roughly,  half  the  velocity  of  the  imping- 
ing jet.  To  get  high  economy  in  the  use  of  steam,  large 
pressure  drops  are  used  and  very  high  jet  velocities  result. 
When  the  buckets  of  a  turbine  are  operated  at  peripheral 
speeds  equal  to  half  these  jet  velocities  one  of  two  diffi- 
culties is  often  met.  The  stresses  induced  in  the  wheel 
structure  by  centrifugal  effects  become  so  high  as  to  offer 
serious  difficulties  in  design,  or  the  rotative  speed  of  the 
unit  becomes  too  high  for  direct  connection  to  the  machine 
which  is  to  be  driven. 

One  method  of  partly  overcoming  the  latter  difficulty 
is  to  operate  the  turbine  at  or  near  the  theoretically  desir- 
able speed  and  transmit  the  power  to  the  driven  machine 
through  gears  which  decrease  the  rotative  speed  to  the 
necessary  extent.  This  method  was  used  with  all  of  the 
early  De  Laval  turbines  which  were  of  comparatively  small 
capacity.  It  is  now  being  successfully  applied  to  marine 
propulsion  and  other  purposes  for  which  large  units  are 
used.  It  is  only  a  partial  remedy  in  the  case  of  large  units, 
however,  as  the  gears  necessary  for  the  desired  reduction 
and  the  size  of  the  turbine  wheels  would  both  become 
excessive, 


THE  STEAM  TURBINE 


239 


240  STEAM  POWER 

Another  and  very  common  method  is  known  as  com- 
pounding or  staging.  This  may  be  of  two  varieties.  The 
pressure  drop  in  each  stage  may  be  limited  to  that 
which  will  give  a  reasonable  velocity  and  a  number  of 
such  stages  may  be  put  together  in  series  on  one  shaft. 
This  would  give  one  set  of  nozzles  and  a  wheel  for  each 
stage,  the  steam  discharged  from  one  wheel  with  very 
low  velocity  expanding  to  a  lower  pressure  through  the 
nozzles  of  the  next  stage  and  impinging  upon  the 
wheel  of  that  stage  with  the  resultant  high  velocity. 
Such  an  arrangement  is  known  as  pressure  staging  or 
pressure  compounding,  and  is  extensively  used  in  large 
turbines. 

The  pressure  staging  method  is  illustrated  in  Fig.  159 
as  applied  to  the  De  Laval  type  of  impulse  turbine.  The 
combined  increase  in  diameter  of  wheels  and  increase 
in  length  of  blades  gives  the  necessary  increase  in  area  to 
pass  the  larger  volumes  of  steam  as  the  drop  of  pressure 
continues  from  stage  to  stage. 

Instead  of  staging  on  a  pressure  basis,  staging  on  a  veloc- 
ity basis  may  be  used.  In  such  a  case  the  drop  in  pressure 
through  one  set  of  nozzles  is  great  and  the  resultant  veloc- 
ity high.  The  steam  moving  at  this  high  velocity  is  then 
directed  upon  the  buckets  moving  at  such  peripheral  velocity 
that  they  absorb  only  part  of  the  kinetic  energy  of  the  steam, 
discharging  it  with  a  lower  absolute  velocity  than  that 
with  which  it  entered,  but  one  which  is  too  high  to  be 
thrown  away.  The  steam  then  passes  through  a  set  of 
stationary  vanes  which  direct  it  upon  the  blades  of  a  second 
wheel,  in  passing  through  which  it  gives  up  still  more  of 
its  kinetic  energy  with  a  corresponding  further  decrease 
of  velocity.  If  the  velocity  still  possessed  by  the  steam 
warrants  it,  a  second  set  of  stationary  guide  vanes  and  a 
third  set  of  moving  buckets  can  be  supplied  for  further 
reducing  it  and  by  carrying  this  velocity  staging  through 
a  sufficiently  great  number  of  stages  any  initial  velocity 


THE  STEAM  TURBINE 


241 


could  be  absorbed  theoretically  without  the  use  of  wheels 
with    high    peripheral    speeds.     Practically,    losses    due   to 


friction,   eddying  and   other  sources  limit  the  number  of 
velocity  stages  to  two  or  three. 


242 


STEAM  POWER 


FIG.  160.— Early  Form  of  Curtis  Turbine. 


THE   STEAM  TURBINE  243 

Velocity  staging  is  combined  with  pressure  staging 
in  the  Curtis  type  of  turbine.  A  section  through  part 
of  an  early  design  of  vertical  turbine  of  this  type  is  shown 
in  Fig.  160.  The  turbine  illustrated  had  four  pressure  stages 
and  each  pressure  stage  had  two  velocity  stages. 

Many  varieties  of  impulse  turbines  have  been  developed 
and  all  of  the  larger  ones  employ  several  wheels  and  sets 
of  nozzles  and  diaphragms  to  obtain  the  necessary  staging. 
The  same  result  has  been  obtained  in  some  of  the  smaller 
models  by  discharging  the  steam  from  nozzles  on  to  a  set 
of  buckets  which  are  able  to  absorb  only  a  fraction  of  the 
kinetic  energy,  catching  it  at  discharge  and  returning  it 
for  another  passage  through  the  buckets,  and  so  on  until 
the  greatest  practical  fraction  of  the  kinetic  energy  has  been 
absorbed. 

106o  The  Reaction  Type.  If  high-pressure '  steam  or 
other  fluid  be  forced  into  a  de- 
vice arranged  as  shown  in  Fig. 
161  and  free  to  revolve  about 
a  vertical  axis,  the  jets  blowing  out 
of  the  nozzles  will  cause  the  mecha- 
nism to  revolve  in  the  direction 
indicated  by  the  arrow.  This  rota-  FIG.  161. 

tion  is  said  to  be  due  to  the  reaction        Elementary  Reaction 
of  the  jets,  and  the  mechanism  there-  Turbine, 

fore  constitutes  a  simple  form  of  reaction  turbine.  By 
increasing  the  number  of  nozzles 
any  amount  of  steam  could  be  dis- 
charged and  therefore  any  amount 
of  work  could  be  obtained. 

This   multiplication   of    nozzles 
can,  however,  be  more  conveniently 
accomplished    by    fastening   radial 
JTIG  162.  vanes  to  the  periphery  of  a  wheel 

as  shown   in   Fig.    162,    the  space 
between  any  two  vanes  constituting  a  nozzle  through  which 


244 


STEAM   POWER 


FIG.  163. 


the  steam  can  discharge.  By  mounting  such  a  wheel 
within  a  casing  as  shown  in  Fig.  163  it  forms  a  simple 
reaction  turbine.  One  of  the  characteristic  differences 
between  the  impulse  and  the  reaction 
types  lies  in  the  distribution  of  pressures. 
In  the  impulse  type  the  nozzles  are 
fastened  into  a  stationary  part  of  the 
turbine  and  the  drop  of  pressure  occurs 
entirely  within  the  nozzles.  The  wheels 
are  therefore  immersed  in  a  space  in 
which  a  uniform,  low  pressure  exists. 
In  the  reaction  type,  on  the  other  hand, 
the  nozzles  are  carried  on  the  wheel  and 
there  must  be  a  higher  pressure  on  one  side  of  the  wheel 
than  there  is  on  the  other.  Since  there  must  also  be  me- 
chanical clearance  between  the  blade  tips  and  the  interior 
of  the  casing,  it  follows  that  the  reaction  type  will  be 
handicapped  by  considerable  leakage  which  does  not  exist 
in  the  impulse  type,  excepting  as  some  of  the  jet  may 
"  spill  "  over  the  ends  of  the  blades. 

The  difference  of  pressure  on  the  two  sides  of  the  wheel 
also  causes  a  tendency  toward  motion  of  the  wheel  along 
the  shaft,  or  of  the  wheel  and  shaft,  in  a  direction  away  from 
the  higher  pressure. 

Many  unsuccessful  efforts  have  been  made  to  design 
efficient  reaction  turbines,  but  no  pure  reaction  type  has 
yet  been  commercialized.  The  turbines  commonly  called 
reaction  turbines  are  really  combinations  of  reaction  and 
impulse  types. 

One  example  of  what  is  commercially  called  a  reaction 
turbine  is  shown  in  Fig.  164.  Alternate  rings  (or  rows) 
of  stationary  and  movable  blades  guide  the  steam  as  it 
expands  from  the  high  pressure  at  one  end  to  the  low  pres- 
sure at  the  other.  The  stationary  blades  project  inward 
from  the  interior  surface  of  the  stationary  casing  and  the 
movable  blades  project  outward  from  the  external  surface 


THE  STEAM  TURBINE 


245 


246 


STEAM  POWER 


of  the  cylindrical  rotor.  The  rotor  blades  act  like  those 
of  an  impulse  turbine  in  partly  reversing  the  direction  of  jets 
of  steam  which  reach  them  with  comparatively  high  veloci- 
ties, but  they  also  act  like  the  movable  nozzles  of  a  reac- 
tion turbine  since  the  steam  in  passing  through  them  expands 
and  acquires  kinetic  energy,  giving  a  reaction  on  discharge. 
The  stationary  blades  serve  to  redirect  the  steam  so  that 
it  strikes  the  next  set  of  moving  blades  at  the  proper  angle 

and  they  also  serve  as 
nozzles  in  which  velocity 
energy  is  acquired.  This 
is  shown  diagrammatical ly 
in  Fig.  165,  in  which  S 
denotes  stationary,  and  M 
movable  blades. 

The  Parsons  type,  il- 
lustrated in  Fig.  164,  may 
be  described  as  a  multistage 
type  in  which  impulse  and 
reaction  are  utilized  in  con- 
junction. 

The  balance  pistons 
shown  in  the  figure  are 
used  to  balance  the  end 
thrust  caused  by  the  differ- 
ence in  pressure  existing  on 
opposite  sides  of  the  wheels 
in  the  case  of  reaction  turbines.  Each  piston  is  of  such 
a  diameter  that  it  presents  a  surface  equal  to  the  blade 
surface  acted  upon  by  one  of  the  unbalanced  pressures, 
and  by  connecting  across  as  shown  in  the  figure  a  high 
degree  of  balance  is  secured. 

The  overload  valve  is  used  to  admit  high-pressure 
steam  to  the  low-pressure  blades  for  carrying  excessive 
overloads.  The  larger  area  of  the  passages  through  these 
blades  permits  an  abnormal  amount  of  high-pressure  steam 


THE  STEAM  TURBINE  247 

to  pass,  thus  giving  a  high  load-carrying    capacity   with 
decreased  economy. 

107.  Combined  Types.     The  clearance  at  the  ends  of 
the  stationary  and  moving  blades  in  the  Parsons  type  of 
turbine  permits  considerable  steam  to  leak  by,  as  previously 
explained.     This    clearance   must    have    almost   the    same 
length  (measured  from  blade  tip  to  opposing  metal)  in  all 
stages  in  order  to  insure  freedom  from  rubbing,  but  it  is 
more  detrimental  in  the  high-pressure   stages  than  in  the 
low.     The  high-pressure  blades  are  much  shorter  than  the 
low-pressure  blades  and  a  leakage  length  of  a  certain  amount 
is  therefore  equal  to  a  greater  fraction  of  the  total  blade 
length.     The  density  of  the  high-pressure  steam  is  also  so 
much  greater  than  that  of  the  low-pressure  steam  that  many 
more  pounds  can  leak  through  an  opening  of  a  given  size 
in  a  given  time.     In  discussions  of  this  character,  it  should 
not  be  forgotten,  however,  that  leakage  area  is  determined 
by  the   dimension   already  referred   to  multiplied   into  a 
circumference  and  that  the  circumference  is  much  greater 
at  the  lower  end. 

Because  of  these  and  other  reasons  many  manufacturers 
have  come  to  the  conclusion  that  the  impulse  type  is  best 
for  the  high-pressure  end  of  the  turbine  and  the  reaction 
type  for  the  low-pressure  end.  Many  such  combinations 
have  been  produced  and  they  are  giving  very  good  results. 

108.  Economy    of    Steam   Turbines.     In    general,    the 
economies  of  steam  turbines  and  reciprocating  engines  are 
about  the  same  when  each  type  is  operated  at  normal  load 
and  under  the  best  conditions.     It  is  probable  that  very 
large  turbines  have  a  slight  advantage  over  reciprocating 
engines  (as  generally  built)  in  the  matter  of  economy  and 
the  reverse  of  this  statement  appears  to  be  true  for  most 
small  units,  although  very  economical  turbines  have  been 
produced  in  small  sizes  in  the  past  few  years. 

The  turbine,  however,  generally  gives  a  flatter  water- 
rate  curve  than  does  a  reciprocating  engine;  that  is,  for 


248  STEAM  POWER 

loads  each  side  of  the  most  economical  the  steam  per  horse- 
power hour  does  not  increase  above  the  value  attained 
at  most  economical  load  as  rapidly  in  the  case  of  turbines 
as  it  does  in  the  case  of  most  reciprocating  engines.  With 
a  very  variable  load,  therefore,  or  with  a  load  which  is  far 
removed  from  the  rated  value,  the  turbine  probably  gives 
a  better  average  performance  than  does  the  reciprocating 
engine.  This  is  particularly  true  in  large  sizes. 

It  has  been  shown  that  the  turbine  operates  on  the  complete- 
expansion  cycle  and  it  will  be  remembered  that  the  recipro- 
cating engine  operates  on  a  cycle  with  incomplete  expansion. 
The  turbine  is  therefore  able  to  make  better  use  of  very 
Ipw-pressure  steam  than  can  the  piston  type. 

Trial  on  a  T</>  or  Mollier  chart  will  show  that  a  turbine 
receiving  steam  at  about  atmospheric  pressure  and  expand- 
ing it  down  to  a  vacuum  of  from  28  to  29  ins.  should  make 
available  as  much  work  as  one  receiving  steam  at  a  high 
boiler  pressure  and  expanding  down  to  atmospheric.  In 
other  words  a  drop  of  100  Ibs.  or  more  above  atmospheric 
pressure  makes  no  more  energy  available  than  does  a  drop 
of  about  13  Ibs.  below  atmospheric,  or  the  lower  the  initial 
pressure  the  more  heat  is  converted  into  work  by  a  given 
pressure  drop.  A  small  decrease  in  back  pressure  (terminal 
or  condenser  pressure)  is  therefore  very  effective  in  the  case 
of  turbines.  Tests  show  that  an  increase  of  one  inch  of 
vacuum  will  cause  an  increase  of  economy  of  from  3  to  10 
per  cent,  depending  upon  the  type  of  turbine  and  upon 
other  factors. 

Experience  has  shown  that  reciprocating  engines  are 
fully  the  equal  of  turbines  in  the  high-pressure  ranges, 
in  many  cases  they  are  even  superior,  but  the  turbine  is  far 
superior  in  the  low-pressure  region  and  in  cases  where  very 
great  ratios  of  expansion  are  to  be  used.  Advantage  has 
been  taken  of  the  superior  ability  of  the  turbine  to  handle 
low-pressure  steam  by  constructing  mixed  plants,  recipro- 
cating engines  being  used  for  expanding  down  to  the  neigh- 


THE  STEAM  TURBINE  249 

borhood  of  atmospheric  pressure  and  turbines  expanding 
the  steam  exhausted  by  these  engines  to  the  lowest  vacuum 
which  can  be  maintained  economically.  This  system  has 
been  found  particularly  useful  for  increasing  the  capacity 
of  a  reciprocating-engine  plant.  The  capacity  of  such  a 
plant  can  often  be  almost  doubled  without  any  increase 
in  boiler  capacity  by  simply  inserting  turbines  into  the 
exhaust  lines  between  the  engines  and  the  condensers,  and 
then  arranging  the  pressures  so  that  the  turbines  carry 
the  expansion  from  about  16  Ibs.  absolute  down  to  a  vacuum 
of  from  28  to  29  ins.  Turbines  used  in  this  way  are  called 
low-pressure  or  exhaust-steam  turbines. 

Superheat  is  also  very  effective  in  bettering  turbine 
economy,  every  ten  degrees  of  superheat  generally  causing 
a  saving  of  about  1  per  cent  in  the  weight  of  steam  required 
per  horse-power. 

The  steam  turbine  is  generally  cheaper  than  the  recipro- 
cating engine  of  like  capacity  if  the  conditions  of  operation" 
permit  the  use  of  the  high  rotative  speed  characteristic 
of  the  turbine.  It  is  therefore  extensively  used  for  direct 
connection  to  blowers,  centrifugal  pumps  and  electrical 
machinery.  Most  of  the  larger  electric  power  stations 
which  have  been  installed  within  the  past  few  years  have 
used  turbines  to  drive  the  generators,  and  single  units 
direct  connected  to  20,000  K.W.  generators  are  now  numer- 
ous. Units  rated  at  45,000  K.W.  are  also  being  installed 
and  units  of  still  larger  capacity  have  been  designed. 

PROBLEMS 

1.  A  steam  turbine  produces  one  horse-power  hour  at  its  shaft 
for  every  30  Ibs.  of  steam  supplied.     The  initial  pressure  is  200 
Ibs.  absolute  and  the  steam  is  superheated  200°  F.     The  turbine 
exhausts  against  a  back  pressure  of  14  Ibs.  absolute. 

Find  the  thermal  efficiency  on  the  assumption  that  heat 
of  liquid  at  exhaust  temperature  is  not  chargeable  to  the  turbine. 

2.  Develop   a   complete   expansion   cycle   for   one    pound   of 
material  used  under  the  conditions  of  Prob.  1  and  find  the  energy 


250  STEAM  POWER 

made  available  per  cycle.  From  this  value  determine  the  number 
of  pounds  of  material  theoretically  required  per  horse-power  hour 
and  compare  with  the  value  given  in  Prob.  1. 

3.  Find  the  additional  quantity  of  energy  which  would  theoret- 
ically be  made  available  per  pound  of  steam  in  above  problems  if 
the  back  pressure  could  be  lowered  to  \  Ib.  absolute. 

4.  Develop  a  complete  expansion  cycle  from  an  initial  pressure 
of  225  Ibs.  absolute  with  a  superheat  of  200°  F.  to  a  back  pressure 
of  |  Ib.  absolute.     Assume  that  this  is  to  be  divided  up  into  six 
parts,  each  making  available  the  same  quantity  of  energy.     Find 
the  pressure  drop  for  each  part.     Note  that  this  is  most  easily 
done  with  the  help  of  the  Mollier  chart. 

5.  A  steam  turbine  receives  steam  at  a  pressure  of  225  Ibs. 
per  square  inch  absolute  and  with  a  superheat  of  190°  F.  and 
exhausts  into  a  condenser  in  which  a  pressure  of  f  Ib.  per  square 
inch  absolute  is  maintained.     The  turbine  is  direct  connected  to 
an  electric  generator  and  produces  a  K.W.-hour  on  12  Ibs.  of  steam. 
If  a  K.W.-hour  is  equivalent  to  3411  B.t.u.,  what  is  the  thermal 
efficiency  of  the  combination? 

6.  Develop  a  complete  expansion  cycle  for  the  conditions  of 
Prob.  5  and  determine  the  pounds  of  steam  which  would  be  re- 
quired theoretically  to  develop  energy  equivalent  to  1  K.W.-hour. 
Compare  with  the  value  given  in  Prob.  5. 

7.  Determine  the  velocity  theoretically  attainable  by  expanding 
steam  in  one  step  from  the  initial  to  the  final  conditions  of  Prob. 
5  above.     What  would  be  the  value  of  the  kinetic  energy  of  such 
a  jet  per  pound  of  steam  flowing? 

8.  Determine   the   shape   of   a   nozzle   required   to   discharge 
1000  Ibs.  of  steam  per  hour,  initial  conditions  being  100  Ibs.  per 
square  inch  absolute,  and   dry  saturated  steam;    final  pressure 
being  2  Ibs.  absolute. 

9.  Determine  velocity  and  kinetic  energy  of  jet  in  Prob.  8. 


CHAPTER  XIV 
CONDENSERS  AND   RELATED   APPARATUS 

109.  The  Advantage  of  Condensing.  The  lowest  pres- 
sure which  can  be  used  in  a  steam-engine  cylinder,  that  is 
the  exhaust  pressure,  is  determined  by  the  pressure  prevail- 
ing in  the  space  into  which  the  steam  is  exhausted.  With  a 
given  initial  pressure  the  amount  of  work  which  can  be  ob- 


FIG.  166. 

tained  theoretically  from  a  given  weight  of  steam  increases 
as  the  exhaust  or  back  pressure  decreases,  as  shown  by  the 
areas  of  the  two  diagrams  in  Fig.  166,  and  experience  has 
shown  that  at  least  a  part  of  this  theoretical  increase  can  be 
obtained  in  real  engines.  It  is  therefore  desirable  to  ex- 
haust into  a  space  in  which  the  lowest  possible  pressure 
exists  when  the  work  obtained  per  pound  of  steam  is  the 
only  consideration. 

The  most  available  space  into  which  an  engine  can 

251 


252  STEAM  POWER 

exhaust  is  that  surrounding  the  earth  and  already  occupied 
by  the  earth's  atmosphere.  The  pressure  in  this  space 
is  approximately  equal  to  14.7  Ibs.  per  square  inch  at  sea 
level  and  is  due  to  the  weight  of  the  atmosphere.  Since 
the  superincumbent  column  of  atmosphere  decreases  in 
depth  as  one  moves  upward,  its  weight  also  decreases  and 
atmospheric  pressure  therefore  averages  less  than  14.7 
Ibs.  per  square  inch  at  high  altitudes  and  has  a  greater 
average  value  at  points  below  sea  level. 

If  it  is  desired  to  exhaust  into  a  pressure  lower  than 
atmospheric  a  means  of  maintaining  such  an  abnormal 
pressure  within  some  sort  of  vessel  must  be  devised.  It 
is  the  purpose  of  a  condenser  and  its  associated  apparatus 
to  make  available  a  space  in  which  such  a  low  pressure  can 
be  maintained.  Its  method  of  operation  will  be  considered 
in  later  sections. 

There  is  also  another  advantage  which  may  be  ob- 
tained by  the  use  of  a  condenser.  It  often  happens  that 
the  water  available  is  not  well  adapted  to  use  in  boilers. 
It  may  be  salt  water  as  in  marine  practice,  or  it  may  contain 
a  number  of  undesirable  gases  and  solids  in  solution  as  often 
occurs  in  stationary  practice.  Some  types  of  condensing 
apparatus  are  so  arranged  that  the  steam  exhausted  from  the 
engine  is  converted  into  liquid  water  without  admixture 
and  can  therefore  be  returned  to  the  boiler  as  practically 
pure  water,  thus  largely  eliminating  the  troubles  that 
would  ensue  from  the  use  of  poor  feed  water. 

110.  Measurement  of  Vacuums.  Assume  that  some 
non-volatile  liquid,  that  is,  a  liquid  that  did  not  vaporize, 
could  be  found  and  also  that  it  contained  no  gases  in  solu- 
tion. If  a  long  tube  were  inserted  in  a  vessel  filled  with 
such  a  liquid  and  had  its  upper  end  connected  with  some 
form  of  vacuum  pump  which  could  remove  air  from  its 
interior,  as  shown  in  Fig.  167,  liquid  would  rise  in  the  tube 
as  the  air  was  removed.  Removal  of  air  would  result  in 
lowering  the  pressure  within  the  tube,  but  the  constant 


CONDENSERS  AND  RELATED  APPARATUS   253 


atmospheric  pressure  on  the  liquid  surface  outside  the  tube 

would  then  force  liquid  up  the  tube  to  such  a  height  that 

the  pressure  pa  of  air  in  the  tube 

plus  the  pressure  due  to  the  column 

of  liquid   of   height  h    within   the 

tube  just  equaled  the  pressure  due 

to  the  atmosphere   on  the   surface 

of  the  liquid  in  the  vessel.     If  the 

pump  could  remove  all  of  the   air 

from  the  tube,  liquid  would  rise  to 

such   a   height    that    the   pressure 

exerted  by  it  on  a  plane   passing 

through    the     lower    surface    just 

equa-led  that  of  the  external  atmos- 
phere. 

The  same  result  could  be  at- 
tained by  using  a  tube  closed  at 

one  end,  filling  it  with  the  liquid, 

and  then  inverting  so  that  the  end 

rested  in  the  liquid  as    shown   in 

Fig.  168.     If  the    tube  were   long 

enough,  the  liquid  would  drop  to  some  such 
point  as  shown,  under  which  conditions  the 
height  of  liquid  would  just  balance  atmos- 
pheric pressure.  This  would  only  be  true  if 
the  liquid  did  not  volatilize  and  did  not  contain 
gases  in  solution;  with  these  assumptions  the 
space  above  the  liquid  in  the  tube  would  con- 
tain absolutely  nothing.  This  space  would  be 
said  to  be  perfectly  vacuous,  or  a  perfect  vacuum 
would  be  said  to  exist  in  that  part  of  the  tube. 
A  device  of  this  character  is  used  to 
measure  the  pressure  of  the  atmosphere  and 
is  known  as  a  barometer.  Mercury  is  used 

as  the  liquid  because  its  high  density  makes  it    possible 

to  use  a  short  tube  and  because  it  may  be  considered 


FIG.  167. 


FIG.  168. 


254 


STEAM  POWER 


as  non-volatile  at  ordinary  temperatures.  The  average 
atmospheric  pressure  at  sea  level,  equal  to  14.7  Ibs.  per 
square  inch  approximately,  can  support  about  30  ins. 
of  mercury,  so  that  this  figure  is  generally  taken  as  the 
standard  sea  level  barometer  reading.  An  atmospheric 
pressure  of  less  than  14.7  Ibs.  would  be  shown  by  a  barom- 
eter reading  of  less  than  30  ins.;  a  greater  atmospheric 
pressure  by  more  than  30  ins.  Corresponding  values  of 
atmospheric  pressure  and  barometer  reading  are  given 
in  Table  VII.  To  this  have  also  been  added  the  altitudes 
to  which  the  different  values  would  correspond  if  a  pressure 
of  14.7  Ibs.  existed  at  sea  level  and  there  were  no  variations 
of  atmospheric  pressure  excepting  those  due  to  change  of 
elevation.  Values  of  this  type  can  only  be  roughly  approx- 
imate, because  local  barometric  variations  are  constantly 
occurring  and  the  sea-level  atmospheric  pressure  varies 
both  sides  of  14.7  Ibs. 

TABLE  VII 
ATMOSPHERIC  PRESSURE,  BAROMETER  READING  AND  ALTITUDE 

(Negative  signs  mean  distance  below  sea  level.) 


Barometer, 
Inches  of  Mercury. 

Atmospheric  Pressure, 
Pounds  per  Square  Inch. 

Altitude, 
Feet  (Approximate). 

25.00 

12.27 

4750 

26.00 

12.76 

26.50 

13.01 

3250 

27.00 

13.25 

27.50 

13.49 

2250 

28.00 

13.74 

28.50 

13.98 

1300 

29.00 

14.23 

29.25 

14.35 

29.50 

14.47 

450 

29.75 

14.60 

30.00 

14.72 

Sea  level 

30.25 

14.84 

30.50 

14.96 

-450 

30.75 

15.09 

31.00 

15.21 

-900 

CONDENSERS  AND  BELATED  APPARATUS      255 

The  exact  value  of  standard  atmospheric  pressure 
at  sea  level  is  taken  at  29.921  ins.  of  mercury,  which  is 
equal  to  14.696  Ibs.  per  square  inch  and  corresponds  to 
the  760  mm.  of  mercury,  used  by  scientists  as  standard. 

A  tube  with  both  ends  open  and  arranged  as  shown 
in  Fig.  167  can  be  used  to  measure  the  degree  of  vacuum 
existing  in  the  space  to  which  its  upper  end  is  connected, 
and  many  vacuum  gauges  are  constructed  on  this  principle, 
using  mercury  as  the  liquid.  The  extent  to  which  the 
pressure  is  lowered  in  the  top  of  the  tube  is  indicated  by 
the  height  to  which  the  mercury  column  rises  and  this 
height  in  inches  is  used  as  a  measure  of  the  vacuum.  Thus 
if  a  perfect  vacuum  were  created  and  if  the  atmospheric 
pressure  were  equal  to  T4.7  Ibs.  the  gauge  would  show 
about  30  ins.  of  mercury  above  the  level  in  the  reservoir. 
If  the  vacuum  were  less  perfect  the  gauge  would  show  a 
shorter  column. 

It  should  be  noted  that  the  reading  of  the  vacuum 
gauge  does  not  give  the  pressure  existing  in  the  vacuous 
space,  but  gives  the  amount  by  which  the  pressure  has  been 
reduced  below  that  of  the  atmosphere,  the  difference 
being  expressed  in  inches  of  mercury.  By  subtracting  this 
reading  from  the  existing  atmospheric  pressure  expressed 
in  the  same  units,  the  absolute  pressure  in  the  partially 
vacuous  space  (expressed  in  inches  of  mercury)  is  obtained. 

It  is  obvious,  therefore,  that  a  vacuum-gauge  reading 
of  say  28  ins.  of  mercury  does  not  always  mean  the  same 
absolute  pressure.  With  a  barometer  reading  of  28  ins. 
it  would  represent  a  perfect  vacuum;  with  a  barometer 
reading  of  30  ins.  it  would  represent  a  partial  vacuum,  the 
absolute  pressure  in  the  partially  vacuous  space  being 
equal  to  2  ins.  of  mercury. 

111.  Conversion  of  Readings  from  Inches  of  Mercury 
to  Pounds  per  Square  Inch.  It  is  often  necessary  to  con- 
vert readings  of  pressure  in  inches  of  mercury  into  pounds 
per  square  inch.  This  can  be  done  with  sufficient  accuracy 


256  STEAM  POWER 

under   ordinary   circumstances   by  multiplying   the  inches 
of  mercury  by  the  constant  0.4908.     Thus, 

Barometer  in  inches  X 0.4908  =  atmospheric 

pressure  in  pounds  per  square  inch       .     .     .     (74) 
and 

(Barometer  in  inches  —  vacuum  gauge  in  inches) 
X  0.4908  =  absolute  pressure  in  partially 
vacuous  space  in  pounds  per  square  inch.  .  (75) 

ILLUSTRATIVE   PROBLEM 

A  vacuum  gauge  constructed  like  that  shown  in  Fig.  167 
reads  27  ins.  when  the  barometer  reads  29.5  ins.  What  is  the 
absolute  pressure  in  the  partial  vacuum  above  the  mercury? 

The  absolute  pressure  is  equal  to  29.5-27=2.5  ins.  of  mer- 
cury, which  is  equal  to 

2.5X0.4908  =  1.227  Ibs.  per  square  inch. 

112.  Principle  of  the  Condenser.  A  perfect  vacuum 
could  be  created  in  any  closed  vessel  with  impenetrable 
walls  if  a  pump  could  be  devised  which  could  remove  all 
material  contained  within  that  vessel.  Or,  any  degree  of 
vacuum  can  be  maintained  in  any  partially  closed  vessel 
by  fitting  it  to  a  pump  which  can  remove  all  material 
flowing  into  the  vessel  as  fast  or  faster  than  it  enters,  raise 
the  pressure  of  this  material  to  atmospheric  or.  higher  and 
discharge  it. 

The  latter  principle  is  made  use  of  in  real  condensers, 
a  pump  of  some  form,  or  an  equivalent,  removing  from  the 
condenser  the  material  exhausted  by  the  engine  and  in- 
leakage  from  the  atmosphere,  and  discharging  it  at  atmos- 
pheric pressure  at  a  sufficiently  rapid  rate  to  maintain 
the  desired  vacuum.  If  the  condenser  and  connections 
can  be  made  leak  proof,  the  pump  or  equivalent  has  to  handle 
only  the  material  exhausted  from  the  engine. 

'A  steam  engine  exhausts  a  mixture  of  steam,  water 
and  gases,  the  gases  being  a  mixture  of  those  originally 


CONDENSERS  AND  RELATED  APPARATUS   257 

dissolved  in  the  boiler-feed  water  and  air  which  leaks  into 
those  parts  of  the  system  in  which  a  partial  vacuum  is  main- 
tained. If  the  pump  had  to  handle  the  same  volume  of 
material  as  is  exhausted  by  the  engine,  no  gain  of  work 
would  result  from  condensing,  because  the  pump  would 
have  to  do  at  least  as  much  work  in  raising  the  pressure 
of  this  material  to  atmospheric  and  discharging  it  as  could 
be  obtained  by  allowing  it  to  expand  in  the  engine. 

Steam,  however,  occupies  a  much  larger  volume  than 
water  at  the  same  temperature  and  pressure.  Thus  steam 
at  212°  F.  occupies  a  volume  of  about  26.79  cu.ft.  per 
pound,  but  water  at  the  same  temperature  and  pressure 
occupies  a  volume  of  only  about  0.0167  cu.ft.  per  pound; 
at  a  temperature  of  120°  F.  which  is  often  used  in  condensers, 
the  specific  volume  of  steam  is  about  203  and  that  of  water 
only  0.0162.  Therefore,  if  the  steam  is  condensed  after 
exhaust  from  the  engine  and  before  entering  the  pump  to 
be  discharged  to  atmosphere,  the  pump  work  is  greatly 
reduced.  The  volume  of  the  condensate  is  almost  negli- 
gible in  comparison  with  the  volume  of  steam  exhausted, 
and  the  work  of  pumping  it  is  almost  negligible  in  compari- 
son with  the  work  it  made  available  in  the  engine. 

Gases  contained  in  the  exhaust  steam  cannot  be  lique- 
fied and  must  be  pumped  as  gases.  The  work  required 
to  pump  them  can,  however,  be  reduced  by  lowering  their 
temperature  as  far  as  possible. 

The  condenser  equipment  may  be  regarded  as  con- 
sisting of  a  combination  of  a  partially  closed  vessel  and 
some  form  of  pump.  The  vessel  is  so  constructed  that  a 
low  temperature  can  be  maintained  within  it  and  that 
large  quantities  of  heat  can  be  removed  from  it  for  the 
purpose  of  condensing  the  exhaust  steam  and  of  cooling  the 
contained  gases.  This  is  generally  done  by  using  large 
quantities  of  cool  water. 

The  absolute  pressure  within  the  condenser  is  made 
up  of  two  parts.  The  two  parts  are,  (a)  that  due  to  the 


258  STEAM  POWER 

water  vapor,  since  the  space  over  the  condensed  water  will 
always  be  filled  with  saturated  steam  at  the  same  tempera- 
ture (approximately)  as  that  of  the  water,  and  (6)  that  due 
to  any  gases  present. 

The  pressure  of  the  saturated  steam  (water  vapor) 
can  be  found  from  the  steam  tables  opposite  the  temperature 
existing  in  the  condenser  and  it  is  the  pressure  which  would 
exist  in  the  condenser  of  an  ideal  system  in  which  no  gases 
were  mixed  with  the  working  substance.  The  pressure 
of  the  gases  can  be  found  by  subtracting  from  the  total 
measured  pressure  in  the  condenser  the  pressure  exerted 
by  the  water  vapor  as  shown  in  the  steam  tables.  The 
pressures  exerted  by  the  water  vapor  and  gases  are  spoken 
of  as  partial  pressures,  since  their  sum  makes  up  the  total 
pressure  within  the  condenser. 

The  presence  of  gases  causes  a  two-fold  loss.  First, 
it  increases  the  pressure  against  which  the  engine  has  to 
exhaust,  thus  raising  the  back-pressure  line  on  the  diagram 
and  decreasing  the  work  area.  Second,  it  increases  the 
work  which  must  be  done  by  the  pump  which  otherwise 
would  only  pump  the  condensate  and  such  saturated  water 
vapor  as  accompanied  it. 

113.  Types  of  Condensers.  The  condensers  actually 
used  in  steam  plants  can  be  roughly  divided  into  two  types, 
as 

(a)  Contact  condensers  and 

(b)  Non-contact  condensers. 

In  the  first  type  the  water  which  is  used  for  condensing 
and  cooling  it  intimately  mixed  with  the  exhaust  from  the 
engine  within  the  condensing  vessel,  and  the  resultant 
mixture  of  condensing  water,  condensate  and  gases  is  drawn 
out  of  this  vessel  and  discharged  to  atmosphere  by  the 
pump. 

In  the  second  type  condensing  water  flows  on  one  side 
of  metal  surfaces  of  some  sort  and  the  exhaust  is  led  over 
the  other  side,  the  heat  being  transmitted  through  the 


CONDENSERS  AND  RELATED  APPARATUS   259 


Condons 


metal.  In  condensers  of  this  type  the  condensate  and 
gases  are  not  mixed  with  the  condensing  water  and  the 
condensate  can  therefore  be  returned  to  the  boiler  as  feed 
water  with  the  advantages  already  mentioned. 

114.  The  Jet  Condenser.  One  of  the  earliest  forms  of 
contact  condensers  which  is  still  very  widely  used  for 
moderate  vacuums  is  commonly 
known  as  the  jet  condenser.  The 
principle  of  operation  of  the  jet 
condenser  is  shown  in  Fig.  169. 
Water,  under  pressure,  entering 
as  indicated,  is  broken  up  into  water  in 
fine  streams  or  jets  and  sprayed 
into  the  exhaust  coming  from 
the  engine.  The  resultant  mix- 
ture flows  downward  into  the 
neck  of  the  condensing  vessel  or 
"  condenser  head  "  and  is  re- 
moved by  some  form  of  pump. 
This  pump  handles  gases,  vapors 
and  water  and  is  known  as 
a  vacuum  pump,  a  wet-vacuum 
pump,  or  a  wet-air  pump,  the 
term  wet  signifying  that  it  han- 
dles the  water  as  well  as  the 


^Mixture  to  Puinp 

FIG.  169. — Jet  Condenser. 


gases. 

The  pressure  within  such  a 
condenser  head  would  be  theo- 
retically equal  to  that  corresponding  to  the  temperature 
of  the  resultant  mixture  if  no  gases  were  present.  In 
practice  the  pressure  of  the  water  vapor  would  roughly 
correspond  to  the  average  temperature  near  the  top  of  the 
vessel  and  there  would  be  a  partial  pressure  due  to  gas 
as  well.  This  gas  would  consist  of  that  brought  over  by 
the  engine  exhaust  plus  that  released  from  the  condensing 
water  under  the  low  pressure  within  the  condenser. 


260 


STEAM  POWER 


Details  of  a  complete  jet  condenser  and.  of  the  method 
of  connecting  it  to  an  engine  are  given  in  Fig.  170.  The 
atmospheric  relief  valve  is  installed  in  all  condensing  sys- 
tems and  is  arranged  to  open  automatically  to  atmosphere 
if  the  pressure  within  the  system  rises  to  atmospheric,  that 
is,  if  the  "  vacuum  is  lost." 


FIG.  170. — Jet  Condenser  and  Method  of  Connecting  to  Engine. 

With  the  jet  condenser  the  pressure  might  start  to 
rise  because  of  slow  action  or  even  stoppage  of  the  pump. 
As  the  condenser  head  filled  up  the  rising  water  would 
ultimately  entirely  cover  the  jet  and  condensation  would 
then  practically  cease.  In  the  arrangement  shown  in 
Fig.  170  there  is  an  additional  safety  device  which  breaks 
the  vacuum  in  the  exhaust  system  if  the  water  in  the  head 


CONDENSEES  A^'D  RELATED  APPARATUS   261 

rises  above  a  certain  height,  thus  preventing  the  external 
atmospheric  pressure  from  forcing  this  water  back  along 
the  exhaust  pipe  and  into  the  cylinder,  an  event  which 
would  probably  result  in  a  wrecked  engine. 

The  jet  condenser  here  described  is  known  as  a  parallel- 
flow  type,  because  everything  within  the  condensing  vessel 
flows  in  the  same  direction.  The  gases  and  vapors  handled 
by  the  pump  theoretically  have  the  same  temperature 
as  that  of  the  mixture  with  which  they  flow  out  at  the 
bottom  of  the  condenser  head.  The  temperature  of  this 
mixture  therefore  determines  the  temperature  of  the  gases 
and  vapors  pumped. 

There  are  numerous  forms  of  contact  condensers  which 
more  or  less  closely  resemble  the  types  of  jet  condenser 
just  described.  They  are  occasionally  all  classed  as  jet 
condensers,  but  more  often  are  given  distinguishing  names. 

One  very  common  form  of  contact  condenser  is  generally 
known  as  a  barometric  condenser.  It  consists  essentially 
of  a  condenser  head,  similar  to  that  used  with  the  jet  con- 
denser already  described,  and  a  tail  pipe  or  barometric 
pipe  which  partly  or  wholly  takes  the  place  of  the  wet- 
vacuum  pump  by  removing  part  or  all  of  the  mixture  formed 
within  the  condenser.  One  model  of  such  a  condenser  is 
shown  in  Figs.  171  and  172. 

The  exhaust  from  the  engine  enters  the  head  through 
the  large  pipe  shown  and  divides  into  two  parts,  one  part 
passing  down  through  the  center  of  the  head  and  the  re- 
mainder flowing  downward  in  the  annular  space  A.  The 
condensing  or  injection  water  enters  as  shown  and  is  divided 
by  the  spraying  cone  and  injected  into  the  engine  exhaust, 
which  enters  the  central  tube  of  the  condenser.  The 
mixture  thus  formed  flows  downward  and  finally  meets  the 
discharge  from  the  lower  end  of  the  annular  space  A,  which 
is  then  condensed.  The  mixture  of  injection  and  con- 
densing water  together  with  such  gases  as  have  been  en- 
trapped, then  flows  downward  into  the  tail  pipe,  which  is 


262 


STEAM  POWER 


over  34  ft.  in  length  and  which  dips  into  the  "  hot  well 
at  its  lower  end.     As  atmospheric  pressure  can  only  sup 


/To  Vacuum  or 
Dry  Air  Pump 


Air  Cooler 


Exhaust 


Water  for  Cooling 
/         "Air" 


FIG.  171. — Barometric  Condenser. 


port  a  column  of  water  about  34  ft.  high,  the  tail  pipe  forms 
an  automatic  wet-vacuum  pump,  water  flowing  from  it  as 
rapidly  as  it  accumulates  within  it. 


CONDENSERS  AND  RELATED  APPARATUS      263 


264 


STEAM  POWER 


Atmospheric  Relief  Valve 


Experience  has  shown  that  the  maintenance  of  a  high 
vacuum  with  this  type  of  condenser  depends  upon  the  ex- 
tent to  which  gases  are  removed  from  the  condenser  head. 
These  gases  are  generally  called  air,  as  the  greater  part  of 
them  is  air.  In  the  type  illustrated  such  "  air  "  as  is 
not  trapped  by  the  descending  mixture  rises  through  the 

hollow  spraying  cone,  then 
through  the  air  cooler  and 
flows  out  through  the  pipe 
indicated  to  the  vacuum  or 
dry-air  pump.  The  air  in 
rising  through  the  center 
of  the  spraying  cone  is 
cooled  by  the  water  flowing 
around  it,  and  it  is  further 
cooled  by  coming  into  con- 
tact with  water  as  it  works 
its  way  through  the  air 
cooler.  This  results  not 
only  in  lowering  its  tem- 
perature, but  also  in  caus- 
ing the  condensation  of  a 
great  deal  of  the  water 
vapor  accompanying  it. 
This  condensed  vapor  col- 

FIG.  173.— Baragwanath  Barometric     |ects  in  the  SPace  surround- 

Condenser.  mg    the    air     cooler     and 

flows  down  into  the  head 

through  the  drain  shown.  The  vacuum  pump,  therefore, 
handles  cool  gases  containing  little  water  vapor  and  prac- 
tically no  liquid  water.  It  is  sometimes  called  a  dry-air 
pump  or  dry- vacuum  pump  for  this  reason. 

The  entrainer  shown  in  the  exhaust  system  in  Fig.  172 
is  so  shaped  that  water  collecting  in  the  exhaust  piping 
and  flowing  into  the  entrainer  is  picked  up  by  the  exhaust 
steam  and  carried  into  the  condenser. 


CONDENSERS  AND  RELATED  APPARATUS      265 

The  flow  of  steam  and  injection  water  in  this  condenser 
is  parallel,  but  the  material  on  its  way  to  the  dry-vacuum 
pump  flows  upward  and  the  cooling  water  flows  downward 
so  that  counter-current  flow  is  used  in  this  part  of  the  appa- 


BARAGWANATH 

CONDENSER 

ORDINARY 

SETTING 

| 

£. 


t 


Engine 


B 


15 


I1 
R 


SYPHONING  ITS 

WATER  FROM 
TANK  OR  FLUME 


FIG.  174. 


ratus.  This  has  the  advantage  of  bringing  the  air  leaving 
the  condenser  into  contact  with  the  cooling  water  just  as 
it  enters  and  therefore  when  it  has  its  lowest  temperature. 

A   somewhat   similar   condenser,    arranged   so   that   it 
requires  no  pump,  is  shown  in  Figs.  173  and  174  (a)  and  (6). 


266 


STEAM  POWER 


Exhaust  and  injection  water  mix  as  shown,  the  quantity 
of  injection  water  being  regulated  by  the  hand  wheel  on 
top  of  the  condenser.  The  mixture  flows  downward  through 
the  narrow  neck  and  the  velocity  attained  in  this  part  of  the 
tail  pipe  is  so  high  that  all  air  and  similar  gases  are  swept 
along  with  the  current. 

For  starting,  the 
valve  V  in  Fig.  174  (b) 
is  opened,  allowing  water 
to  flow  into  the  lower 
part  of  the  tail  pipe. 
This  creates  a  partial 
vacuum,  and  atmos- 
pheric pressure  then 
forces  water  up  the  in- 
jection pipe  and  into 
the  condenser  head.  The 
valve  V  is  then  closed 
and  the  condenser  con- 
tinues to  siphon  its  own 
water.  Because  of  this 
action  this  type  is  often 
called  a  siphon  conden- 
ser. By  supplying  a 
circulating  pump  as  in- 
dicated in  Fig.  174  (a) 
it  can  be  converted  into 
a  barometric  condenser 

similar  to  the  type  already  discussed  except  for  the  fact 
that  it  requires  no  air  pump. 

The  barometric  or  tail  pipe  of  any  barometric  condenser 
can  be  replaced  by  any  kind  of  a  pump,  and  centrifugal 
pumps  are  often  used  for  this  purpose.  When  large  quanti- 
ties of  gas  are  to  be  handled,  as  when  a  dry-air  pump 
is  not  used,  the  centrifugal  pump  must  be  specially 
designed. 


FIG.  175. — Westinghouse-Leblanc  Air 
Pump. 


CONDENSERS  AND  RELATED  APPARATUS      267 

A  recently  developed  type  of  condenser  in  which  the 
barometric  tube  is  replaced  by  a  centrifugal  pump  and  in 
which  a  separate  air  pump  of  a  rotary  type  is  used  is  illus- 
trated in  Figs.  175,  176  and  177.  It  consists  essentially  of 
the  condensing  head  and  well,  combined  with  a  centrifugal 
tail  pump  and  a  rotary  air  or  vacuum  pump  as  indicated 


Submerged  not 
less  thau  3  ft. 


FIG.  176. — Westinghouse-Leblanc  Condenser. 


in  Fig.  177.  Injection  water  entering  through  nozzles  in 
the  head  meets  the  exhaust,  and  the  resultant  mixture 
flows  down  into  the  well  through  the  large  nozzle  shown. 
The  liquid  is  continuously  removed  from  the  bottom  of  this 
well  by  the  centrifugal  tail  pump  and  discharged  to  the  hot 
well.  The  air  and  associated  vapors  are  drawn  down  the 
air  pipe  and  discharged  by  means  of  the  device  shown  in 


268 


STEAM  POWER 


Fig.  175.  Water  enters  the  central  part  of  this  pump  as 
indicated  in  Fig.  176  and  is  discharged  through  the  station- 
ary nozzles  N  and  the  moving  vanes  V  shown  in  Fig.  175. 
The  water  is  thus  caused  to  form  a  series  of  "  pistons  " 
which  move  rapidly  downward  in  the  discharge  nozzle  N' 
and  which  trap  small  plugs  or  lamina  of  "  air  "  between 

them  and  thus  discharge 
the  "  air  "  to  the  atmos- 
phere. The  connection 
marked  P  is  used  for  prim- 
ing at  starting  when  neces- 
sary. 

In  small  units  the  cen- 
trifugal tail  pump  may  be 
omitted  and  the  design  so 
remodeled  that  all  the  injec- 
tion water  passes  through 
the  rotary  air  pump  which 
discharges  the  entire  mix- 
ture from  the  condenser 
just  as  it  discharges  the  air 
and  associated  vapors  in 
the  larger  sizes. 

115.  Non-contact  Con- 
densers. The  type  called 
the  surface  condenser  is 
the  best-known  example  of 
non-contact  condenser.  It 

consists  essentially  of  a  large  cylindrical  or  rectangular 
vessel  into  which  the  exhaust  is  discharged  and  through 
which  pass  numerous  bronze  or  alloy  tubes  which  carry  the 
condensing  water,  and  the  surfaces  of  which  act  as  the 
condensing  and  cooling  surface. 

One  form  of  surface  condenser  mounted  above  the 
pumps  which  serve  it  is  shown  in  Fig.  178.  The  exhaust 
enters  at  the  top  of  the  rectangular  shell  and  works  its 


.Water  to 
Air  Pump 


FIG.  177. — Westinghouse-Leblanc 
Condenser. 


CONDENSERS  AND  RELATED  APPARATUS   269 


270  STEAM  POWER 

way  down  over  the  water-cooled  tubes.  The  condensate, 
mixed  with  gases  and  vapors,  is  drawn  from  the  bottom  of 
the  shell  by  the  wet-vacuum  pump  and  discharged  to  the 
hot  well. 

The  condensing  water  is  forced  through  the  tubes 
of  the  condenser  by  means  of  the  reciprocating  circulating 
pump,  entering  the  lower  tubes  at  the  right-hand  end  in 
the  figure,  making  two  "  passes  "  through  the  condenser  and 
leaving  at  the  top.  Because  of  the  path  of  the  water  a 
condenser  of  this  type  is  sometimes  called  a  two-pass  or 
double-flow  condenser. 

With  the  arrangement  illustrated,  the  steam  which 
condenses  upon  the  upper  tubes  falls  as  a  rain  from  tube 
to  tube  until  it  finally  settles  at  the  bottom  and  is  drawn 
off.  The  outer  surfaces  of  the  lower  tubes  are  therefore 
practically  covered  with  water  and  this  has  two  disad- 
vantages. First,  these  tubes  carry  the  coolest  circulating 
water  and  they  therefore  cool  the  condensate  coming  in 
contact  with  them  while  the  water  flowing  through  them 
is  unnecessarily  heated.  Cooling  of  the  condensate  means 
a  lower  hot-well  temperature  than  would  otherwise  be 
obtained,  but  if  the  condensate  is  to  be  used  for  boiler 
feed,  the  temperature  of  water  in  the  hot  well  should  be 
maintained  as  high  as  possible, since  this  water  will  eventually 
have  to  be  heated  to  boiler  temperature  with  a  correspond- 
ing expenditure  of  heat.  Second,  tubes  which  are  being 
used  to  heat  water  covering  them  are  of  little  use  as  condens- 
ing surface,  and  hence  a  large  part  of  the  surface  in  such  a 
condenser  is  comparatively  inactive. 

The  ideal  arrangement  would  carry  away  the  liquid 
condensate  as  fast  as  formed,  leaving  the  tubes  first  entered 
by  the  condensing  water  to  act  as  the  final  condensing 
and  cooling  surfaces,  thus  bringing  gases  and  non-condens- 
ible  vapors  into  contact  with  the  coolest  surfaces  j  ust  before 
entering  the  vacuum  pump.  Numerous  designs  which 
approximate  this  ideal  have  been  developed  recently  and 


CONDENSERS  AND  RELATED  APPARATUS   271 

they  give  better  results  than  do  the  earlier  and  simpler 
forms.  The  improvement  is  shown  by  the  values  of  con- 
densing surface  per  developed  horse-power  of  engine.  In 
early  designs  it  was  customary  to  supply  2|  sq.ft.  of  tube 
surface  or  more  per  horse-power.  Some  of  the  most  recent 
installations  are  giving  better  vacuums  with  only  1  sq.ft. 
per  horse-power. 

One  of  the  newer  models  passes  the  condensate  through 
a  set  of  tubes  so  located  that  the  engine  exhaust  strikes 
them  before  impinging  on  any  tubes  carrying  condensing 
water.  This  results  in  a  partial  condensation  of  the  exhaust 
and  raises  the  temperature  of  the  condensate  within  the 
tubes  to  very  near  that  of  the  exhaust,  thus  heating  the 
boiler  feed  to  a  temperature  practically  corresponding 
to  the  exhaust  temperature  of  the  engine. 

Surface  condensers  are  commonly  operated  with  a 
vacuum  of  from  24  to  26  ins.  of  mercury  when  used  with 
reciprocating  engines  and  with  a  vacuum  of  28  to  29  ins. 
when  receiving  the  exhaust  of  steam  turbines.  When 
operated  at  the  lower  vacuums  wet-vacuum  pumps  are  gen- 
erally used,  but  the  best  types  of  dry-air  pumps  must  be 
installed  in  combination  with  well-designed  condensers 
when  the  higher  vacuums  are  sought. 

116.  Water  Required  by  Contact  Condensers.  The 
weight  of  circulating  water  required  varies  with  the  type 
of  condenser  and  with  the  conditions  of  operation,  such  as 
initial  temperature  of  water,  vacuum  desired,  etc.  It  can  be 
determined  approximately  by  calculation  and  the  values 
thus  found  must  then  be  increased  by  such  factors  as 
experience  has  shown  to  be  necessary. 

In  contact  condensers  the  water  and  the  condensate 
are  discharged  as  a  mixture  and  therefore  have  the  same 
average  discharge  temperature. 

Let  ti  =  initial  temperature  of  injection  water  in  F.°; 
t2  =  temperature  at  which  mixture  is  discharged  in 


272  STEAM  POWER 

X  =  total  heat  above  32°  F.  of  steam  as  exhausted; 
W  =  pounds  of  injection  water  per  pound  of  exhaust 
steam. 

Assuming  the  exhaust  steam  to  be  dry  saturated,  each 
pound  of  steam  in  condensing  to  water  at  a  temperature 
of  fa  degrees  must  give  up  an  amount  of  heat  equal  to  X 
minus  the  heat  of  the  liquid  at  t°2  or  roughly  X—  (fa  —  32) 
B.t.u.  This  same  quantity  must  be  absorbed  by  the  in- 
jection water,  while  its  temperature  rises  from  t\  to  fe 
degrees.  Each  pound  of  water  can  then  absorb  approxi- 
mately (fa  —  ti)  B.t.u.  and  the  pounds  of  injection  water 
per  pound  of  steam  will  be 


(78) 


The  value  of  fe  would  be  that  corresponding  to  the 
absolute  pressure  in  the  condenser  if  it  were  not  for  the 
air  and  similar  gases  which  exert  some  pressure.  It  is 
generally  10  or  more  degrees  F.  below  the  temperature 
corresponding  to  the  vacuum.  Values  of  t%  in  the  neigh- 
borhood of  110°  to  125°  F.  are  customary  with  recipro- 
cating engines  and  values  as  low  as  80°  are  used  with  high 
vacuums  in  connection  with  steam  turbines. 

The  weight  of  water  used  per  pound  of  steam  as  given 
by  Eq.  (78)  will  vary  between  about  15  for  very  low  initial 
and  moderate  discharge  temperature  to  about  50  with 
average  initial  and  moderate  discharge  temperature.  Ex- 
perience shows  that  it  is  necessary  to  add  10  per  cent  or 
more  to  the  values  of  W  obtained  from  equation  (78)  to 
obtain  the  weight  of  water  which  will  probably  be  used. 

ILLUSTRATIVE   PROBLEM 

Find  the  quantity  of  water  theoretically  required  per  pound 
of  steam  condensed  in  a  contact  condenser  in  which  a  vacuum 
of  25.5  ins.  of  mercurv  is  maintained  when  the  barometer  reads 


CONDENSERS  AND  RELATED  APPARATUS   273  . 

29.5  ins.  of  mercury.  The  initial  temperature  of  the  water  is 
60°  F. 

The  absolute  pressure  in  the  condenser  is  29.5-25.5=4.0  ins. 
of  mercury  and  the  steam  tables  give  for  this  pressure,  X  =  11  15.0 
and  ti  =  126.  Substituting  in  Eq.  (78)  gives 

w     1115.0-126+32 

126  -60~     =         approximately. 

117.  Weight  of  Water  Required  by  Non-contact  Con- 
densers. In  the  case  of  non-contact  condensers  there  is 
no  definite  relation  between  the  discharge  temperature 
of  the  cooling  water  and  that  of  the  condensate.  Experi- 
ence shows  that  the  discharge  temperature  of  the  circulating 
water  is  generally  from  10  to  20  degrees  lower  than  the 
temperature  corresponding  to  the  vacuum. 

The^temperature  of  the  condensate  (hot-well  tempera- 
ture) is  generally  15  or  more  degrees  below  that  correspond- 
ing to  the  vacuum,  but  good  design  makes  the  hot-well 
temperature  very  closely  approximate  that  corresponding 
to  the  vacuum. 

Assuming 

ti  =  initial  temperature  of  injection  water  in  F.°; 

£2  =  final  temperature  of  injection  water  in  F.°; 

tc  =  temperature  at  which  condensate  is  discharged,  i.e., 

hot-  well  temperature,  in  F.°; 
X  =  total  heat  above  32°  F.  of  steam  as  exhausted, 

and 

W  =  pounds   of   injection   water   per   pound   of   exhaust 
steam. 

The  weight  of  water  which  must  be  circulated  per  pound 
of  steam  can  be  found  as  in  the  case  of  the  contact  con- 
denser. It  is  given  by 


(79) 

12  —  h 


274  STEAM  POWER 

Values  in  the  neighborhood  of  25  Ibs.  of  water  per  pound 
of  steam  are  common  with  low  vacuums  and  50  or  more 
pounds  are  often  used  with  vacuums  over-  28  ins.  of 
mercury. 

ILLUSTRATIVE   PROBLEM 

A  surface  condenser  receives  circulating  water  at  a  temper- 
ature of  65°  F.  and  discharges  it  at  a  temperature  of  80°  F.  It 
maintains  a  vacuum  of  28.0  ins.  with  the  barometer  at  29.5,  and 
the  temperature  of  the  condensate  discharged  to  the  hot  well  is 
equal  to  85°  F.  Find  the  quantity  of  circulating  water  theoretically 
required. 

This  vacuum  corresponds  to  an  absolute  pressure  of 
29.5—28.0  =  1.5  ins.  of  mercmy.  Assuming  this  all  due  to  steam 
(neglecting  presence  of  air)  the  value  of  X  may  be  taken  from  the 
steam  table  as  1100.1  B.t.u.  Substitution' in  Eq.  (79)  then  gives 

1100.1-85+32 

"  =  ~~  ~^ — 7^- =69.9  approximately. 

80— bo 

118.  Relative  Advantages  of  Contact  and  Surface  Con- 
densers. The  contact  types  are  as  a  rule  much  cheaper 
than  the  surface  condensers,  and  they  are  less  subject  to 
serious  depreciation,  the  tubes  of  surface  condensers  often 
corroding  seriously  in  very  short  intervals  of  time.  On 
the  other  hand,  the  injection  of  the  cooling  water  into 
the  condensing  space  in  contact  types  results  in  the  intro- 
duction of  large  quantities  of  dissolved  gases,  and  much 
of  this  material  is  liberated  under  the  reduced  pressure, 
thus  tending  to  increase  the  condenser  pressure,  that  is, 
decrease  the  vacuum.  Where  pumps  are  used  to  carry 
away  the  mixture  with  contact  condensers,  these  pumps 
have  to  handle  a  much  larger  quantity  of  water  than  the 
corresponding  pump  in  a  surface  condenser,  and  the  work 
of  pumping  this  water  out  of  the  vacuum  into  the  atmos- 
phere combined  with  the  additional  work  required  of 
the  pump  which  handles  the  "  air  "  may  partly  balance 
the  advantage  of  lower  first  cost  of  the  contact  type. 


CONDENSERS  AND  RELATED  APPARATUS   275 

A  surface  condenser  must  always  be  installed  where 
it  is  desirable  to  use  the  condensate  as  boiler  feed,  and 
it  is  generally  used  when  very  high  vacuums  (low  absolute 
pressures)  are  to  be  maintained.  The  surface  condenser 
is  at  a  serious  disadvantage,  however,  when  required  to 
handle  the  exhaust  of  reciprocating  engines.  The  exhaust 
from  such  engines  always  contains  large  quantities  of 
lubricating  oil  carried  out  of  the  cylinder,  and  unless  this 
material  is  separated  before  the  exhaust  enters  the  con- 
denser it  is  deposited  on  the  outer  surfaces  of  the  tubes 
and  decreases  the  conductivity  of  those  surfaces.  Such 
oil  can  be  eliminated  to  a  great  extent  before  the  exhaust 
enters  the  condenser  by  means  of  oil  separators,  which  are 
generally  made  up  of  a  series  of  baffles  upon  which  the 
steam  impinges  and  upon  which  the  oil  is  caught. 

119.  Cooling  Towers.  The  large  quantity  of  circula- 
ting water  required  by  condensing  plants  is  often  an  item 
of  great  economic  importance.  When  such  plants  are 
located  near  a  river  or  near  tide  water,  the  circulating 
water  can  generally  be  procured  for  the  cost  of  pumping. 
When  they  are  located  in  the  middle  of  cities  or  in  regions 
where  water  is  scarce,  the  cost  of  water  may  be  excessive 
or  it  may  even  be  impossible  to  obtain  a  continuous  supply 
equal  to  the  demand  of  the  condensers. 

In  such  cases  the  condensing  water  is  often  circulated 
continuously,  being  cooled  after  each  passage  through  the 
condensers.  This  cooling  is  generally  done  by  exposure 
of  a  large  surface  to  the  air.  The  resultant  evaporation 
of  some  of  the  water  with  the  absorption  of  its  latent  heat 
of  vaporization  cools  the  remainder  so  that  it  can  be  used 
again.  This  sort  of  cooling  may  be  effected  by  running 
the  water  into  a  shallow  pond  of  large  area,  or  by  spraying 
it  into  the  air  over  a  small  pond  or  reservoir  or  by  passing 
it  through  a  cooling  tower. 

Cooling  towers  are  large  wood  or  metal  towers  generally 
filled  with  some  form  of  baffling  devices.  The  hot  water 


276  STEAM  POWER 

is  introduced  at  the  top  and  spread  into  thin  sheets  or 
divided  up  into  drops  as  it  descends.  Air  enters  at  the 
bottom  and  flows  upward,  cooling  the  water  by  contact 
and  by  the  partial  evaporation  which  results.  The  cir- 
culation of  air  may  be  natural,  i.e.,  due  to  the  difference 
of  temperature  between  the  air  inside  and  out,  in  which 
case  a  stack  is  fitted  to  the  top  of  the  tower;  or  the  cir- 
culation may  be  forced  by  fans  located  in  the  base  of  the 
tower.  In  the  latter  case  the  apparatus  is  called  a  forced 
draught  cooling  tower. 


CHAPTER  XV 
COMBUSTION 

120.  Definitions.  Certain  substances  are  known  to 
chemists  as  compounds,  because  they  can  be  separated  by 
chemical  processes  into  simpler  substances.  Thus  water 
and  many  of  the  most  familiar  materials  known  to  man 
are  compounds  which  can  be  separated  into  two  or  more 
simpler  materials. 

Those  substances  which  cannot  be  further  broken  up  by 
the  processes  used  in  separating  compounds  are  called 
elements;  they  are  regarded  as  elemental,  as  the  stones 
of  which  the  compounds  of  nature  are  built  up.  About 
seventy-five  of  these  elements  are  now  known,  but  many 
of  them  are  comparatively  rare.  Pure  metals  are  all 
elements;  the  oxygen  and  nitrogen  which  are  mixed  to  form 
the  greater  part  of  the  atmosphere  are  elements;  carbon, 
which  forms  the  greater  part  of  most  fuels,  is  an  element. 

In  many  cases  the  combination  of  elements  to  form 
compounds  is  accompanied  by  the  liberation  of  heat,  and 
some  of  these  combinations  are  used  by  the  engineer  for  the 
purpose  of  obtaining  heat  in  large  quantities.  When  the 
elements  which  occur  in  fuels,  such  as  coal,  wood  and 
petroleum,  combine  with  oxygen,  the  process  is  spoken  of 
as  combustion.  The  quantity  of  heat  liberated  when  a 
pound  of  any  material  combines  with  oxygen  (burns)  is 
called  the  heat  value  or  calorific  value  of  that  material. 

Fuels  contain  a  great  number  of  elements,  but  only 
three  of  these  ordinarily  take  part  in  combustion  and  are 
therefore  spoken  of  as  combustibles.  They  are  carbon, 
hydrogen  and  sulphur.  The  sulphur  content  is  generally 

277 


278  STEAM  POWER 

very  small,  and  the  carbon  and  hydrogen  are  therefore  the 
most  important  constituents. 

The  combustion  of  each  of  these  elements  will  be  con- 
sidered in  detail  in  the  following  sections,  but  before  this 
can  be  done  two  other  idea's  must  be  developed. 

The  smallest  particle  of  an  element  which  can  be 
conceived  of  as  entering  into  combination  to  form  a  com- 
pound is  known  as  an  atom  of  that  element.  It  has  been 
found  that  the  atoms  of  each  element  have  an  invariable 
and  characteristic  mass.  The  lightest  atom  is  that  of 
hydrogen,  and  its  weight  is  considered  unity.  The  atom 
of  carbon  is  twelve  times  as  heavy  as  that  of  hydrogen  and 
carbon  is  therefore  said  to  have  an  atomic  weight  equal  to 
twelve.  Similarly  the  atomic  weight  of  nitrogen  is  four- 
teen and  that  of  oxygen  is  sixteen. 

The  smallest  particle  which  can  be  formed  by  the  com- 
bination of  atoms  is  known  as  a  molecule.  Like  or  unlike 
atoms  may  combine  to  form  molecules.  Thus  two  hydro- 
gen atoms  combine  to  form  a  molecule  of  hydrogen,  and 
hydrogen  gas  as  it  ordinarily  exists  may  be  pictured  as 
made  up  of  a  collection  of  such  molecules.  Similarly, 
gaseous  oxygen  and  gaseous  nitrogen  may  be  pictured  as 
collections  of  molecules  which  are  made  up  of  two  like 
atoms. 

When  unlike  atoms  combine  to  form  a  molecule,  they 
form  a  molecule  of  a  compound.  Obviously  a  molecule  of 
any  compound  is  the  smallest  particle  of  that  compound 
which  can  exist. 

For  convenience,  the  different  elements  are  represented 
by  abbreviations;  thus  oxygen  is  represented  by  O,  nitro- 
gen by  N,  hydrogen  by  H,  carbon  by  C  and  sulphur  by  S. 
When  these  abbreviations  are  written  in  chemical  equa- 
tions, such  as  will  be  given  later,  they  stand  for  an  atom 
of  the  substance.  Hence  O  in  a  chemical  equation  would 
mean  one  atom  of  oxygen.  The  symbol  O2  is  used  to  mean 
two  atoms  of  oxygen  in  combination,  hence,  one  molecule 


COMBUSTION  279 

of  oxygen.  The  symbol  202  means  two  groups  of  two 
oxygen  atoms  in  combination,  hence  two  molecules  of  oxygen. 
The  simplicity  and  elegance  of  this  system  will  become 
apparent  as  the  chemical  equations  which  follow  are  de- 
veloped and  explained. 

121.  Combustion  of   Carbon.     Carbon  can  unite  with 
oxygen  or  burn  to  form  two  different  compounds  —  carbon 
monoxide  (CO)  and  carbon  dioxide  (C02).     The  monoxide 
is  formed  by  the  combination  of  one  atom  of  oxygen  with  one 
atom  of  carbon;    the  dioxide,  by  the  combination  of  two 
atoms  of  oxygen  with  one  of  carbon.    The  dioxide,  therefore, 
contains  twice  as  much  oxygen  as  does  the  monoxide. 

Carbon  burned  to  carbon  monoxide  has  not  combined 
with  the  largest  possible  quantity  of  oxygen,  and  combus- 
tion is  therefore  said  to  be  incomplete  in  such  cases.  When, 
however,  carbon  dioxide  is  formed,  the  carbon  has  combined 
with  as  much  oxygen  as  possible  and  combustion  is  said  to 
be  complete. 

It  will  be  shown  later  that  much  more  heat  is  liberated 
when  the  dioxide  is  formed  than  when  carbon  burns  to  the 
monoxide.  Hence,  when  liberation  of  heat  is  the  object  of 
combustion,  the  process  should  be  so  conducted  as  to  result 
in  the  formation  of  the  maximum  quantity  of  dioxide  and 
the  minimum  amount  of  monoxide. 

122.  Combustion  to  CO.     The  combustion  of  carbon  and 
oxygen  to  form  the  monoxide  can  be  represented  by  the 
equation 

C+O  =  CO,      ......     (80) 

or  by  the  equation 

2C+O2  =  2CO  .......     (81) 


The  former  is  the  simpler  and  will  be  considered  first,  but 
the  latter  is  the  more  perfect  and  indicates  more  to  the 
trained  eye  than  does  the  simpler  form. 

The  simple  equation  states  that  one  atom  of  carbon 
combined  with  one  atom  of  oxygen  to  form  one  molecule 


280  STEAM  POWER 

of  carbon  monoxide.  It  can,  however,  be  so  interpreted 
as  to  show  much  more  than  this.  The  carbon  atom  is  twelve 
times  as  heavy  as  the  hydrogen  atom,  while  the  oxygen  atom 
is  sixteen  times  as  heavy  as  that  of  hydrogen.  The  equation 

C  +  O=.CO, 

therefore,  shows  that  an  atom, which  is  twelve  times  heavier 
than  the  hydrogen  atom,  unites  with  one  which  is  sixteen 
times  heavier  than  the  hydrogen  atom  to  form  a  molecule 
which  is  28(  =  12  +  16)  times  heavier  than  the  hydrogen 
atom. 

In    other   words,    the   weights  of   carbon   and    oxygen 

12     3      1 

combining  are  in  the  ratio  of  -— =  T  =  TT.     If  a    sufficient 

16     4     1J 

number  of  carbon  atoms  to  weigh  one  pound  be  used,  a 
quantity  of  oxygen  weighing  1|  Ibs.  will  be  required  to 
combine  with  them  to  form  carbon  monoxide.  The  re- 
sultant carbon  monoxide  will  contain  the  pound  of  carbon 
and  the  1J  Ibs.  of  oxygen  and  will  therefore  weigh  2J  Ibs. 

The  same  weight  relations  would  hold  irrespective  of 
the  weight  of  carbon  used,  and  the  simpler  equation  may 
therefore  be  put 

1  weight  of  C+1J  weights  of  O  =  2J  weights  of  CO.     (82) 

ILLUSTRATIVE   PROBLEM 

To  illustrate  the  use  of  this  equation,  assume  that  9  Ibs.  of 
carbon  are  burned  to  carbon,  monoxide  and  that  it  is  desired  to 
find  the  weight  of  oxygen  used,  and  the  weight  of  the  product. 
The  weight  of  oxygen  used  must  be  1^  times  the  weight  of  carbon, 
that  is,  1^X9  =  12  Ibs.  The  weight  of  the  product  must  be  2£ 
times  the  weight  of  the  carbon,  that  is  2|X9  =21  Ibs.;  or,  it  must 
be  the  weight  of  the  carbon  burned  plus  the  weight  of  the  ox}^gen 
used,  that  is,  9+12  =21  Ibs. 

In  general,  the  oxygen  used  for  combustion  is  obtained 
from  the  atmosphere,  which  may  be  considered  as  a  median- 


COMBUSTION  281 

ical  mixture  of  oxygen  and  nitrogen  in  unvarying  porportions. 
These  proportions  are  roughly,  0.23  of  oxygen  to  0.77  of 
nitrogen  by  weightier  0.21  of  oxygen  to  0.79  of  nitrogen 
by  volume,  as  shown  in  Table  VIII.  The  weight  of 
air  which  contains  one  pound  of  oxygen  is  therefore 

0.23+0.77  ,      . 

—  =  4.35    Ibs.,  and    this    weight    of    air    contains 
U..£o 

4.35-1=3.35  Ibs.  of  nitrogen. 

In  the  problem  previously  considered  it  was  found 
that  12  Ibs.  of  oxygen  would  be  required  to  burn  9  Ibs. 
of  carbon  to  CO.  The  total  weight  of  air  required  to 
obtain  this  oxygen  will  be  12X4.35  =  52.2  Ibs.  and  it  will 
contain  52.2-12  =  40.2  Ibs.  of  nitrogen. 

By  simple  arithmetical  calculations  of  the  type  just 
given  all  the  weight  relations  involved  in  the  combustion 
of  C  to  CO  can  be  determined.  The  volume  of  air  required 
in  any  given  case  can  be  found  by  multiplying  the  weight 
of  air  by  the  specific  volume  as  given  in  Table  VIII. 
Thus,  in  the  illustrative  problem  already  considered,  it 
was  found  that  52.2  Ibs.  of  air  would  be  required  to  burn 
9  Ibs.  of  C  to  CO.  The  volume  of  this  air  at  62°  F.  would 
be  52.2X13.14  =  685.9  cu.ft. 

It  is  found  that  a  quantity  of  heat  equal  to  about 
4500  B.t.u.  is  liberated  per  pound  of  carbon  burned  to  CO; 
that  is  the  calorific  value  of  C  burned  to  CO  is  4500  B.t.u. 

Returning  now  to  Eq.  (81),  which  was  said  to  be  more 
useful  than  the  simpler  form  given  as  Eq.  (80),  it  will  be 
necessary  to  consider  a  rather  simple  law  of  gases.  It 
has  been  shown  experimentally  that  equal  volumes  of  all 
gases  contain  the  same  number  of  molecules  when  at  the  same 
temperature  and  pressure.  This  statement  is  known  as 
Avogadro's  Law..  It  has  also  been  shown  that  the  mole- 
cules of  gaseous  oxygen  contain  two  atoms. 

The  equation  in  question, 


282 


STEAM  POWER 


can  therefore  be  read,  two  atoms  of  carbon  combine  with 
one  molecule  of  oxygen  to  form  two  molecules  of  carbon  mon- 
oxide. But,  if  every  molecule  of  O  yields  two  molecules 
of  CO  it  follows  from  Avagadro's  law  that  the  CO  formed 
will  occupy  twice  the  volume  of  the  oxygen  used  if  measured 
at  the  same  temperature  and  pressure.  If  the  equation  be 
imagined  as  containing  a  numeral  1  before  the  O2,  it 
will  be  obvious  that  the  coefficients  of  the  terms  represent- 
ing gas  molcules  give  volume  relations  directly.  This  equa- 
tion therefore  gives  both  volume  and  weight  relations. 


TABLE  VIII 
PROPERTIES  OF  AIR 

Considering  it  to  Consist  only  of  nitrogen  and  oxygen. 


Relative  Proportions. 

Ratio  of  N  to  O. 

Ratio  of  Air  to  O. 

Exact. 

Approx. 

Exact. 

Approx. 

Exact, 

Approx. 

By  weight  .  . 
By  volume.  . 

/0.766N 

10.234O 

/0.791N 
10.209O 

0 

0 

0 
0. 

77  N 
23  O 

79  N 
21  O 

3.27 

3.78 

3.35 
3.76 

4 
4 

27 
76 

4.35 
4.76 

Spec.  wt.  at  Atmos.  Press. 
(Lbs.  per  Cu.ft.) 

Spec.  Vol.  at  Atmos.  Press. 
(Cu.ft.  per  Lb.) 

At  32°  F. 

At  62°  F. 

At  32°  F. 

At  62°  F. 

0.08072 

0.07609 

12.39 

13.14 

Weight  of  air  containing  one  pound  of  oxygen  is  approximately 
4.35  Ibs. 

123.  Combustion  to  CO2.  The  combination  of  carbon 
and  oxygen  to  form  the  dioxide  is  represented  by  the  equa- 
tion 

(83) 


COMBUSTION  283 

which  shows  that  one  atom  of  carbon  (twelve  times  heavier 
than  hydrogen)  combines  with  two  atoms  of  oxygen  (each 
sixteen  times  heavier  than  hydrogen)  to  form  a  molecule 
of  CO2,  which  is  forty-four  times  heavier  than  an  atom  of 
hydrogen.  Therefore  the  weight  of  carbon  and  oxygen 

12        31 

combining  are  as  ^TTTTH  =  o  =  o2>  so  tnat  2t  H>s.  °f  oxygen 
ZXIO     o      Z;j 

are  required  to  burn  a  pound  of  carbon  to  carbon  dioxide. 
Writing  this  in  the  form  of  an  equation,  gives 

1  weight  of  C-f  2|  weights  of  O  =  3f  weights  of  CO2.  .     (84) 

The  weight  of  air  required  can  readily  be  found  by 
multiplying  the  required  oxygen  by  the  number  4.35, 
previously  shown  to  be  the  number  of  pounds  of  air  con- 
taining one  pound  of  oxygen.  Thus,  the  required  air  is 
2f  X4. 35  =  11. 57  pounds  per  pound  of  C  burned  to  C02. 
This  number  is  commonly  rounded  out  to  12  in  engineering 
literature. 

The  equation  given  shows  volume  relations  directly. 
It  is  evident,  therefore,  that  one  molecule  of  O  yields  one 
molecule  of  CCb,  and  hence  that  the  volume  of  the  product 
is  exactly  equal  to  the  volume  of  the  oxygen  used  in  forming 
it  if  measured  at  the  same  temperature  and  pressure. 
This  is  a  very  important  relation,  and  is  often  made  use 
of  in  engineering  calculations. 

Experiment  shows  that  when  carbon  burns  to  the 
dioxide  about  14,600  B.t.u.  are  liberated  per  pound  of 
carbon  burned,  that  is,  the  calorific  value  of  C  burned  to  CO2 
in  14,600. 

124.  Combustion  of  CO  to  CC>2.  Since  carbon  which 
has  burned  to  carbon  monoxide  has  not  combined  with  the 
greatest  possible  quantity  of  oxygen,  the  monoxide  can 
take  up  more  oxygen  by  burning  to  the  dioxide.  This 
process  is  represented  by  the  formula 

2CO+02  =  2CO2,  (85) 


284  STEAM  POWER 

which  shows  that  two  molecules  of  monoxide  combine  with 
one  molecule  of  oxygen  to  form  two  molecules  of  the  dioxide. 
The  volume  of  CO2  formed  is  therefore  equal  to  that  of 
the  CO  burned. 

So  far  as  the  ultimate  result  is  concerned,  at  makes  no 
difference  whether  carbon  is  burned  directly  to  CO2  or  is 
first  burned  to  CO  and  then  the  CO  is  burned  to  CO2 
The  total  oxygen  used  per  pound  of  carbon  burned  to  CO2 
and  the  total  heat  liberated  per  pound  of  carbon  burned 
to  C02  are  the  same  in  both  cases. 

Thus,  for  the  oxygen,  one  pound  of  C  burned  to  CO2 
requires  2f  Ibs.  of  oxygen;  but  one  pound  of  C  burned 
to  CO  requires  1J  Ibs.  of  oxygen,  and  1J  Ibs.  more  will  be 
required  when  this  CO  is  burned  to  C02.  The  result 
is  therefore  the  same. 

For  heat  liberated,  one  pound  of  C  burned  to  C02 
liberates  about  14,600  B.t.u.;  but  one  pound  of  C  burned 
to  CO  liberates  about  4500  B.t.u.  and  10,100  B.t.u.  are 
liberated  when  this  CO  is  burned  to  C02.  Since  the  sum 
of  4500  and  10,100  is  equal  to  14,600  the  result  is  again 
the  same. 

Data  on  the  combustion  of  C  to  CO  and  CO2  and  the 
combustion  of  CO  to  CO2  'are  collected  in  convenient 
form  in  Table  IX. 

125,  Conditions  Determining  Formation  of  CO  and  CO2. 
Excluding  certain  complicated  considerations  which  are 
not  of  great  importance  in  steam-power  engineering,  it  may 
be  said  that  when  carbon  is  being  burned  at  a  certain  rate 
(pounds  per  unit  of  time)  the  amount  of  oxygen  brought 
into  contact  with  the  carbon  determines  whether  the  caibon 
burns  to  CO  or  to  CO2.  If  enough  or  more  than  enough 
oxygen  to  burn  the  carbon  to  CO2  is  brought  into  contact, 
that  oxide  will  be  formed.  If  there  is  not  enough  to  burn 
all  the  carbon  to  the  dioxide,  both  oxides  are  formed  in  cer- 
tain proportions,  which  can  be  calculated. 

Since  combustion  to  CO  yields  only  4500  B.t.u.   per 


COMBUSTION 


285 


pound  of  C  and  combustion  to  €62  yields  14,600  B.t.u. 
per  pound  of  C,  the  importance  of  supplying  sufficient 
oxygen  to  burn  all  carbon  to  the  dioxide  in  cases  where 
the  liberation  of  the  maximum  quantity  of  heat  is  desirable 
is  obvious.  In  actual  practice  the  oxygen  is  furnished 
by  supplying  air  and  it  is  found  necessary  in  most  cases 
to  supply  more  than  the  amount  of  air  theoretically  re- 
quired in  order  to  insure  burning  all,  or  even  nearly  all, 
of  the  carbon  to  the  dioxide.  This  comes  from  the  great 
difficulty  met  in  obtaining  contact  between  the  oxygen  of  the 
air  and  the  carbon  which  is  to  be  burned,  that  is,  in  bringing 
all  the  oxygen  of  the  air  into  intimate  contact  with  the 
fuel  being  burned  in  real  apparatus. 


TABLE  IX 
COMBUSTION  DATA  FOR  CARBON 

(Per  pound  of  carbon  ) 


Oxygen  Required. 

Nitrogen  Accompanying 
Oxygen. 

T>«,   ^a          Cu.ft.  at  62°  F. 
Pounds.          and  14  7  j  bs 

Pounds. 

Cu.ft.  at  62°  F. 
and  14.7  Lbs. 

CO  

1.333                  16.0 

4.46 

60.1 

CO2  from  C. 

2  667              32  0 

8  92 

120  2 

CO,  from  CO  

1.333              16.0 

4.46 

60.1 

Air  Required. 

Quantity  of  Product 
(N  not  included). 

Product. 

Cu.ft.  at 

Cu.ft  at 

ated. 

Pounds. 

62°  F.  and 

Pounds. 

62°  F.  and 

14.7  Lbs. 

14.7  Lbs. 

CO 

5   79 

76.1 

2.33 

32.0 

4,500 

CO2  from  C  

11.58 

152.2 

3.67 

32.0 

14,600 

r 

10,100  per  Ib. 

CO2fromCO.  ... 

5.79 

76.1 

3.67 

32.04 

1 

of  C  in  CO 
4,300  per  Ib. 

of  CO 

286  STEAM  POWER 

The  air  in  excess  of  that  theoretically  required  to  burn 
all  the  carbon  completely  is  spoken  of  as  excess  air.  In 
the  form  of  an  equation,  this  statement  is  equivalent  to 

Air  supplied  — air  theoretically  required  =  excess  air.     (86) 

It  is  customary  to  express  the  quantity  of  excess  air  in 
terms  of  a  numerical  factor  known  as  the  excess  coefficient. 
This  coefficient  is  defined  as  the  number  by  which  the  quantity 
of  air  theoretically  required  must  be  multiplied  to  give  the 
quantity  of  air  actually  used.  In  the  form  of  an  equation 
this  gives 

Excess  coefficient X air  theoretically  required 

=  air  actually  used.    .     (87) 

ILLUSTRATIVE   PROBLEM 

Taking  data  from  the  illustrative  problem  previously  considered, 
assume  that  9  Ibs.  of  carbon  are  burned  in  air  to  C02.  Each  pound 
theoretically  requires  11.57  Ibs.  of  air,  so  that  the  theoretical 
air-supply  for  this  case  would  be  9X11.57=104.13  Ibs.  If  in  a 
real  case  150  Ibs.  of  air  are  supplied,  the  excess  coefficient  is  equal 
to  150^  104. 13  =1.44. 

126.  Flue   Gases  from   Combustion  of  Carbon.      The 

gases  resulting  from  the  combustion  of  fuels  are  known 
in  engineering  as  the  products  of  combustion  or  flue  gases, 
because  they  are  the  gases  passing  through  the  flues  or 
passages  leading  from  furnaces  in  which  fuel  is  burned  and 
to  the  stacks  which  serve  to  carry  off  the  gases. 

It  has  already  been  shown  that  the  CO2  formed  by  the 
combustion  of  carbon  has  the  same  volume  as  the  oxygen 
which  is  used  in  forming  it.  Therefore,  if  the  air  supplied 
in  a  given  case  just  equaled  that  theoretically  required 
for  combustion  to  CO2  and  if  all  of  the  oxygen  were  used, 
the  CO2  formed  would  merely  replace  the  oxygen  in  the 
air.  The  theoretical  proportions  of  the  flue  gas  would 
then  be  0.21  of  CO2  and  0.79  of  N  by  volume, 


COMBUSTION 


287 


If  real  flue  gases  obtained  by  burning  carbon  in  air  are 
found  to  contain  less  than  21  per  cent  of  CO2,  the  combustion 
has  evidently  not  yielded  theoretically  perfect  flue  gases. 
The  trouble  may  be  due  to  an  excess  or  to  a  deficiency  of  air. 
If  there  is  an  excess  of  air  there  will  be  oxygen  present  in 
the  flue  gases;  if  there  is  a  deficiency  there  will  be  CO 
present  in  the  flue  gases.  An  analysis  of  these  gases  for 
oxygen  and  for  CO  would  therefore  indicate  the  source  of 
trouble  and  the  remedy  to  be  provided. 

The  curve  to  the  right  of  the  central  vertical  line  in 
Fig.  179  shows  the  theoretical  decrease  in  volume  per 


30 

oj30 

1" 

£2° 

8  15 

> 
I. 

°S 
D 

\ 

\ 

\% 

\ 

/ 

s^ 

\ 

[Xx 

\ 

^ 

\ 

\ 

*? 

-  — 

-  —  ,  , 

Z 

\ 

0       40      30      20       10        0       50      100      150     200    250    301 
eflciency  (in  per  cent)         Excess  (In  per  cent) 
1234 

FIG.  179.— Effect  of  Air  Supply  on  Flue  Gas  Analysis. 

cent  of  C02  in  flue  gases  as  the  excess  air  increases.  The 
single  numbers  1,  2,  3  and  4  indicate  the  excess  coeffi- 
cients corresponding  to  the  various  percentages  of  excess 
air. 

The  curves  to  the  left  give  the  theoretical  decrease  in 
volume  per  cent  of  CO2  and  the  theoretical  increase  in 
volume  per  cent  of  CO  as  the  air  supplied  is  decreased  below 
that  theoretically  required  for  complete  combustion. 

127.  Combustion  of  Hydrogen.  Hydrogen  combines 
with  oxygen,  or  burns,  to  fonn  water.  The  equation  for 
this  reaction  is 

2H2+02  =  2H20,  ......     (88) 


288  STEAM  POWER 

which  indicates  that  two  molecules  of  hydrogen  combine 
with  one  molecule  of  oxygen  to  form  two  molecules  of 
water.  In  terms  of  volumes,  two  volumes  of  hydrogen 
combine  with  one  of  oxygen  to  form  two  of  gaseous  water, 
that  is,  water  in  the  form  of  highly  superheated  vapor. 
4s  the  water  is  cooled  down  it  will  obviously  approach 
and  finally  reach  the  liquid  condition,  with  a  rapid  de- 
crease in  volume  quite  different  from  that  experienced 
by  a  gas  under  similar  conditions,  so  that  the  volume  rela- 
tions hold  only  at  high  temperatures. 

The  weight  relations  can  be  calculated  as  in  ether 
cases,  starting  from  the  fact  that  four  weights  of  hydrogen 
combine  with  thirty-two  weights  of  oxygen  to  form  36 
weights  of  water.  The  weights  of  hydrogen  and  oxygen 
are  therefore  in  the  relation  of  -g^  =  J. 

The  heat  liberated  when  one  pound  of  hydrogen  burns 
to  water  is  equal  to  about  62,000  B.t.u.  This  is  the  quantity 
of  heat  which  could  be  obtained  if  one  pound  of  hydrogen 
at,  say,  room  temperature,  and  mixed  with  the  requisite 
quantity  of  oxygen,  were  ignited  and  the  resultant  water 
were  then  cooled  down  to  the  initial  temperature.  During 
the  cooling  of  the  water  it  would  partly  or  entirely  condense 
and  thus  give  up  some  or  all  of  its  latent  heat  of  vaporization. 
This  heat  would  obviously  be  included  in  the  calorific 
value  just  given. 

In  many  pieces  of  engineering  apparatus  in  which 
hydrogen  is  burned  the  products  of  combustion  are  not 
cooled  to  such  an  extent  that  the  water  is  condensed.  The 
latent  heat  of  vaporization  would  not  be  liberated  under 
such  conditions,  but  would  remain  bound  up  with  the  water 
vapor.  When  the  water  is  not  condensed  the  heat  liberated 
is  only  about  52,000  B.t.u.  per  pound  of  hydrogen.  'This 
number  is  known  as  the  lower  calorific  value  of  hydrogen, 
while  62,000  is  known  as  the  higher  calorific  value. 

Data  on  the  combustion  of  hydrogen  are  given  in 
Table  X. 


COMBUSTION 

TABLE  X 
COMBUSTION  DATA  FOR  HYDROGEN 

(Per  pound  of  hydrogen) 


289 


Product. 

Oxygen  Required. 

Nitrogen  Accompanying  Oxygen. 

Cu.ft  at  62°  F. 
and  14.7  Lbs. 

Pounds. 

Cu.ft.  at  62°  F 
and  14.7  Lbs. 

H2O  

8                            96 

26.8 

361 

Product. 

Air  Required. 

Quantity  of  Product  (N 
not  included). 

Heat 
Liberated. 

Pounds. 

Cu.ft.  at  62° 
F.  and  14.7 
Lbs. 

Pounds. 

Cu.ft.  at  62° 
F.  and  14.7 
Lbs. 

H2O  .-.-' 

34.8 

457 

9 

Liquid 
0.144 

/  62,000 
I  52,000 

128.  Combustion  of  Hydrocarbons.  Many  of  the  fuels 
used  by  the  engineer  contain  compounds  of  hydrogen 
and  carbon  which  are  called  hydrocarbons.  One  of  the  best 
examples  is  methane  (CH4),  which  forms  the  greater  part 
of  all  the  so-called  natural  gas. 

All  of  these  hydrocarbons  burn  to  CO2  and  H^O  if 
the  supply  of  oxygen  is  great  enough.  If  there  is  a  deficiency 
of  oxygen,  combustion  is  incomplete  and  generally  results 
in  the  formation  of  CO2,  H^O,  CO,  C  in  the  form  of  soot, 
and  other  products  which  need  not  be  considered  here. 

For  complete  combustion  the  requisite  oxygen  and 
air  can  be  determined  as  in  previous  cases  by  means  of 
chemical  equations.  Thus  for  methane  the  equation  is 


=  CO2+2H2O, 


(89) 


which  shows  that  sixteen  (12+4)  weights  of  methane 
combine  with  sixty-four  (2X2X16)  weights  of  oxygen  to 
form  forty-four  (12+32)  weights  of  carbon  dioxide  and 
thirty-six  (4+32)  weights  of  water. 


290  STEAM  POWER 

The  calorific  value  of  hydrocarbons  is  generally  assumed 
to  be  equal  to  the  sum  of  the  heat  values  of  the  carbon 
and  hydrogen  contained  in  one  pound  of  the  material. 
Thus,  if  C  represent  the  fraction  of  a  pound  of  carbon 
contained  in  one  pound  of  the  hydrocarbon  and  if  H 
represent  the  fraction  of  a  pound  of  hydrogen  contained 
therein,  the  common  assumption  would  make  the  higher 
calorific  value  of  the  hydrocarbon 

(CX  14,600)  +  (HX  62,000)  B.t.u.       .     .     (60) 

The  results  obtained  in  this  way  do  not  generally  check 
well  with  the  experimentally  determined  values,  and  it  is 
best  to  use  the  latter  when  they  are  available. 

129.  Combustion  of  Sulphur.  Sulphur  forms  several 
different  oxides,  but  when  burned  under  engineering  con- 
ditions it  is  generally  assumed  to  form  only  the  dioxide 
S(>2.  The  chemical  equation  for  such  combustion  is 


(91) 


and  since  the  atomic  weight  of  sulphur  is  32,  this  equation 
shows  that  equal  weights  of  sulphur  and  oxygen  combine 
to  form  the  dioxide. 

The  combustion  of  sulphur  to  862  liberates  about 
4000  B.t.u.  per  pound  of  sulphur. 

130.  Combustion  of  Mixtures.  It  is  often  necessary 
to  obtain  approximate  calorific  values  of  combustible 
materials  which,  without  great  error,  can  be  considered 
as  mixtures  of  combustible  and  non-combustible  elements. 
If  there  is  oxygen  present  in  the  mixture  it  is  assumed 
to  be  combined  with  hydrogen  in  the  form  of  water,  so 
that  the  uncombined  or  available  hydrogen  per  pound  of 
material  is  given  by  the  expression 

AvailableH  =  H--,     .....     (92) 
8 


COMBUSTION  291 

in  which  H  and  O  respectively  represent  the  fractions  of 
a  pound  of  hydrogen  and  oxygen  in  one  pound  of  material. 
The  calorific  values  of  such  a  mixture  containing  car- 
bon, hydrogen  and  sulphur  would  then  be  given  approxi- 
mately by  the  equation 


Higher  B.t.u.  -  14,6000+  62,000  H-    -    +4000S,    (93) 

\         o/ 

in  which  the  letters  stand  respectively  for  the  fractions 
of  a  pound  of  each  of  the  elements  present  in  one  pound  of 
the  mixture.  Similarly  the  lower  calorific  value  would 
be  (approximately) 

Lower  B.t.u.  =  14,600C+  52,000  (  H-^-)  +4000S,     (94) 

\        P/ 
and  the  oxygen  required  will  be 

Pounds  of  O  =  2iC+8(H-^\  +S.  (95) 


131.  Temperature  of  Combustion.  If  combustion  of 
any  material  could  be  carried  on  inside  of  an  ideal  vessel 
which  did  not  absorb  nor  transmit  heat,  the  heat  liberated 
during  the  combustion  could  not  escape  from  the  space 
within  the  vessel. 

If  the  vessel  contained  initially  only  the  combustible 
and  the  oxygen  or  air  required  to  burn  it,  the  products  of 
combustion  would  be  the  only  material  contained  within 
the  vessel  after  the  completion  of  combustion.  Under 
such  circumstances  the  heat  would  be  used  in  raising  the 
temperature  of  the  products  of  combustion,  and  the  process 
could  be  pictured  as  though  all  of  the  combustion  occurred 
first,  forming  the  products  of  combustion  without  change 
of  temperature,  and  then  the  liberated  heat  raised  the 
temperature  of  these  products. 

Knowing  the  weight  of  each  of  these  products  and  the 
quantity  of  heat  required  to  raise  the  temperature  of  one 


292  STEAM  POWER 

pound  of  each  of  them  one  degree,  the  amount  of  heat 
required  to  raise  all  of  them  one  degree  could  be  found  by 
multiplying  the  two  known  values.  Thus,  if  carbon  had 
been  burned  in  oxygen  to  CCb  with  the  theoretical  oxygen 
supply,  the  vessel  would  contain  only  carbon  dioxide. 
To  raise  the  temperature  of  one  pound  of  carbon  dioxide 
one  degree  requires  an  amount  of  heat  equal  to  the  specific 
heat  of  that  gas.  Therefore,  if  W  represents  the  weight 
of  CC>2  formed  and  C  represents  its  specific  heat,  the  amount 
of  heat  required  to  raise  the  temperature  of  all  of  the  CC>2 
one  degree  would  be  W-C  B.t.u.  If  Q  B.t.u.  were  liberated 
by  the  combustion,  the  temperature  rise  in  degrees  would 
therefore  be  given  by 

Temp,  rise-  ^,     .....     (96) 

and  if  the  initial  temperature  had  been  to  degrees,  the  final 
temperature  would  be 

.......     '     (97) 


A  final  temperature  figured  in  this  way  is  called  the  theo- 
retical temperature  of  combustion.  It  can  never  be  attained 
in  practice  because  of  heat  lost  to  surroundings  and  because 
of  other  losses  which  need  not  be  considered  here. 

Theoretical  temperatures  of  combustion  are,  moreover, 
nearly  always  calculated  on  the  assumption  that  the  specific 
heats  of  gases  are  constants,  whereas  they  really  increase 
with  the  temperature.  It  therefore  follows  that  tempera- 
tures determined  on  the  assumption  of  constant  specific 
heat  will  be  too  high  for  this  reason  also. 

When  gases  are  heated  there  are  two  distinctly  different 
limiting  possibilities;  the  volume  occupied  by  the  gases 
may  remain  constant  or  the  pressure  exerted  by  the  gases 
may  remain  constant  while  the  volume  increases.  In 
the  case  of  constant  volume  all  the  heat  added  to  the  gases 


COMBUSTION  293 

must  be  used  for  raising  the  temperature;  the  amount 
of  heat  required  per  pound  per  degree  under  these  con- 
ditions is  known  as  the  specific  heat  at  constant  volume 
and  is  designated  by  Cp. 

When,  however,  the  volume  is  allowed  to  increase  at 
such  a  rate  as  to  keep  the  pressure  constant  the  heat  sup- 
plied must  not  only  raise  the  temperature,  but  must  also 
do  whatever  external  work  is  done  in  displacing  (pushing 
out  of  the  way)  surrounding  mediums.  The  heat  required 
per  pound  per  degree  under  these  conditions  is  known  as 
the  specific  heat  at  constant  pressure  and  is  represented 
by  Cp.  It  is  always  greater  than  Cv  by  the  amount  of  heat 
required  to  do  the  external  work  accompanying  a  rise  of 
temperature  of  one  degree. 

Thus,  in  the  case  assumed  above,  had  the  vessel  been 
so  constructed  that  its  internal  volume  did  not  change, 
the  specific  heat  at  constant  volume  would  be  used.  On 
the  other  hand,  had  the  vessel  been  fitted  with  a  movable 
piston  arranged  to  move  outward  at  such  a  rate  as  to  main- 
tain constant  pressure  within  the  vessel  as  the  temperature 
rose,  the  specific  heat  at  constant  pressure  would  be 
used. 

In  most  cases  the  material  burned  is  not  pure  carbon, 
but  a  fuel  containing  carbon,  hydrogen  and  sulphur,  and  as 
air  is  generally  used  to  furnish  the  oxygen,  the  products 
of  combustion  will  contain  not  only  the  oxides  of  carbon, 
hydrogen  and  sulphur,  but  inert  nitrogen  as  well.  The 
temperature  rise  is  determined  in  the  same  way,  however, 
by  dividing  the  heat  liberated  by  the  amount  required  to 
raise  the  temperature  of  the  products  one  degree.  Thus 
if  Wi,  W2,  Ws  .  .  .  Wn  stand  for  the  weights  of  the  various 
products  and  Ci,  €2,  C3  .  .  .  Cn  for  their  respective  specific 
heats,  the  theoretical  temperature  rise  is  given  by 


'   (98) 


294  STEAM  POWER 

and  the   theoretical  temperature  of  combustion  is  given  by 


3CS  .   .    .    WnCn' 

if  to  stands  for  the  initial  temperature. 

PROBLEMS 

1.  Assume  10  Ibs.  of  C  burned  to  CO.     Determine  the  quan- 
tity of  oxygen  required,  the  quantity  of  air  required,  the  quantity 
of  nitrogen  in  this  air,  and  the  quantity  of  heat  liberated. 

2.  What  will  be  the  volume  of  the  CO  formed  as  above  if 
measured  at  62°  F.  and  14.7  Ibs.  pressure? 

3.  Assume  15  Ibs.  of  C  burned  to  C02.     Determine  the  quan- 
tities of  oxygen  and  air  required,  the  quantity  of  nitrogen  con- 
tained in  this  air,  and  the  quantity  of  heat  liberated. 

4.  What  will  be  the  volume  of  the  C02  formed  from  15  Ibs. 
of  carbon  if  measured  at  62°  F.  and  14.7  Ibs.? 

5.  What  will  be  the  volume  of  the  flue  gases  formed  by  the 
combustion  of  11  Ibs.  of  carbon  to  C02  with  the  theoretical  air 
supply? 

6.  The  quantity  of  CO  obtained  by  the  combustion  of  8  Ibs. 
of  carbon  is  burned  to  C02  with  the  theoretical  amount  of  oxygen. 
Determine  the  quantities  of  oxygen  and  air  required,  the  amount 
of  nitrogen  contained  in  this  air,  and  the  quantity  of  heat  liberated. 

7.  Assume  5  Ibs.  of  C  burned  in  air  to  C02  with  an  excess 
coefficient  of  1.5.     Determine  the  quantities  of  oxygen  and  air 
supplied,  the  heat  liberated  and  the  composition  of  the  flue  gases. 

8.  The  composition  of  flue  gases  resulting  from  the  combus- 
tion of  carbon  in  air  is  found  to  be  21%  of  C02  and  79%  of  N 
by  volume.     What  is  the  value  of  the  excess  coefficient? 

9.  An  analysis  of  flue  gases  resulting  from  the  combustion 
of  carbon  in  air  shows  12%  of  C02  by  volume  and  no  CO.     The 
gases  are  not  analyzed  for  0  or  N.     What  can  you  say  with  regard 
to  the  air  supply? 

10.  Three  pounds  of  hydrogen  burn  with  theoretical  oxygen 
supply.     Determine  the  wreight  of  oxygen  and  air  used,  the  weight 
of  the  resultant  water  and  the  weight  of  the  flue  gas. 

11.  Determine  the  heat  liberated  in  the  preceding  problem  if 
the  water  vapor  is  condensed  and  if  it  is  not  condensed. 

12.  How  much  hydrogen  would  have  to  be  burned  to  obtain 
20  Ibs.  of  water? 


COMBUSTION  295 

13.  The  chemical  formula  of  methane  is  CH4.     If  one  pound 
of  methane  is  burned  with  theoretical  air  supply,  what  weight 
of  air  will  be  used,  and  what  will  be  the  weight  of  the  flue  gases? 

14.  What  would  be  the  percentage  composition  of  the  flue 
gases  of  the  preceding  problem  on  a  weight  basis? 

15.  The  chemical  formula  of  ethane  is  C2H6.     Determine  the 
calorific  value  of  this  material  by  means  of  the  formula  given 
in  the  text. 

16.  A  certain  material  is  found  to  have  the  following  analysis 
on  a  weight  basis:  C,  85%;  H,  12%;  0,  1%;  S,  2%.     Determine 
the  calorific  value  of  this  material  by  means  of  the  formula  given 
in  the  text,  assuming  that  all  the  oxygen  present  is  in  combination 
with  hydrogen. 

17.  Determine  the  amount  of  oxygen  required  to  completely 
burn  3  Ibs.  of  the  material  described  in  the  preceding  problem. 

18.  One  pound  of  carbon  is  burned  to  CO2  in  pure  oxygen 
in  a  vessel  so  arranged  as  to  maintain  constant  internal  pressure. 
The  specific  heat  of  C02  at  constant  pressure  and  at  ordinary 
temperatures  is  about  0.21.     Calculate  the  theoretical  temperature 
rise  and  the  temperature  of  combustion,  using  this  value  of  the 
specific  heat  and  assuming  an  initial  temperature  of  60°  F. 

19.  Make  the  same  calculations  as  called  for  in  the  preceding 
problem,  but  using  the  value  0.27  for  the  specific  heat  of  C02. 
This  is  more  nearly  the  average  value  of  the  specific  heat  over 
the  range  of  temperature  existing  in  such  a  case. 

20.  The  hydrocarbon  ethylene  is  represented  by  the  chemical 
formula  C2H4.     Assume  that  one  pound  of  this  material  is  burned 
in  air  within  a  vessel  arranged  to  maintain  the  products  at  con- 
stant pressure  and  that  the  excess  coefficient  is  1.5.     Determine 
the  theoretical  temperature  of  combustion  if  the  initial  temperature 
is  60°  F.,  the  mean  specific  heat  of  CO2  is  0.27,  that  of  H20  is 
0.61,  that  of  N  is  0.27,  and  that  of  0  is  0.24. 


CHAPTER  XVI 
FUELS 

132.  Commercial  Fuels.  In  engineering  practice  any- 
thing which  is  combustible  and  which  can'  be  procured  in 
large  quantities  at  a  reasonable  cost  is  called  a  fuel.  The 
principal  commercial  fuels  are: 

f(l)  Coal. 

a.  Solid         |  (2)  Wood  and  wood  wastes. 
1(3)  Vegetable  wastes. 

{(1)  Crude  petroleum  or  natural  oil. 
(2)  Various  products  made  from  petroleum. 
(3)  Methyl  and  ethyl  alcohol. 

c    Gaseous    j(1)  Natural  Sas' 

[  (2)  Artificial  or  manufactured  gases. 

Coal  is  by  far  the  most  extensively  used  fuel  because  of 
its  abundance  and  relative  cheapness  in  most  localities. 
However,  in  oil-producing  regions  the  crude  oil  and  some 
of  the  products  made  from  it  are  more  often  the  commonly 
used  fuel,  particularly  if -good  coal  is  not  mined  in  the 
immediate  vicinity. 

Wood  is,  in  general,  too  valuable  to:be  used  exclusively 
as  a  fuel  excepting  on  the  frontiers  where  wooded  terri- 
tory is  being  opened  up  and  where  coal  cannot  be  pro- 
cured excepting  at  prohibitively  high  cost.  Wood  wastes, 
on  the  other  hand,  are  very  often  used  for  fuel  in  the  indus- 
tries producing  them. 

Vegetable  wastes,  like  wood  wastes,  are  essentially  of 
local  value,  being  practically  entirely  consumed  by  the 
industries  producing  them. 

296 


FUELS  297 

Natural  gas  is  in  many  respects  an  ideal  fuel,  and  is 
extensively  used  for  power  production  in  some  localities. 
The  diminution  in  the  quantity  available,  the  consequent 
rise  in  the  price,  the  great  economy  achieved  in  burning 
this  gas  in  gas  engines  and  the  increased  use  of  the  gas  for 
domestic  purposes  are,  however,  gradually  eliminating  this 
fuel  from  the  steam-power  field. 

Artificial  gases  have  never  been  extensively  used  for  the 
generation  of  steam,  as  it  is  generally  cheaper  to  burn  the 
materials  from  which  the  gases  are  made,  rather  than  to 
convert  them  into  gas  and  then  to  burn  the  gas  under 
boilers.  This  condition  may  change  in  the  future  when 
better  markets  have  been  opened  up  for  some  of  the  by- 
products which  can  be  obtained  from  artificial  gas  plants. 

133.  Coal.  The  word  coal  is  used  as  the  name  of  a 
great  group  of  natural  fuels  which  consist  of  more  or 
less  metamorphosed  vegetable  remains.  At  one  end  of  the 
group  is  the  material  known  as  peat,  which  is  only  slightly 
changed  from  the  original  vegetable  substance;  at  the 
other  end  is  the  graphitic  anthracite  which  has  undergone 
such  radical  metamorphosis  that  practically  all  of  the 
original  vegetable  material  excepting  carbon  and  ash  has 
been  eliminated. 

A  common,  rough  classification  of  the  coals  in  the  order 
of  age,  or  of  completeness  of  carbonization  is, 

1.  Peat  or  turf. 

2.  Lignite  (brown  or  black). 

3.  Sub-bituminous  coal. 

4.  Bituminous  coal. 

5.  Semi-bituminous  coal. 

6.  Semi-anthracite. 

7.  Anthracite. 

8.  Graphitic  anthracite. 

The  divisions  are  not  at  all  exact,  as  they  depend  partly  upon 
chemical  composition  and  partly  upon  physical  properties. 


298 


STEAM  POWER 


Another  classification  of  a  more  exact,  variety  is  that 
given  in  Table  XI  and  partly  illustrated  in  Fig.  180,  which 
gives  what  is  known  as  Mahler's  curve.  It  is  for  United 
States  coals  only.  The  terms  used  in  this  classification 
are  explained  in  subsequent  paragraphs. 


16000 


15000 


60  70  80  90 

#Fixe.d  Carbon  In  the  Combustible 


100 


FIG.  180.— Mahler's  Curve  for  United  States  Coals. 


TABLE  XI 

CLASSIFICATION  OF  COALS 


Division. 

Per  cent  of 
Fixed  Carbon 
in  Combustible. 

Per  cent  of 
Volatile  Matter 
in  Combustible. 

Calorific  Value, 
B.t.u.  per  Pound  of 
Combustible. 

Graphitic  
Anthracite  
Semi-anthracite  
Semi-bituminous  
Eastern  bituminous.  .  . 
Western  bituminous.  . 
Lignite  

100     to  97 
97     to  92.5 
92.5  to  87.5 
87.5  to  75 
75     to  60 
65     to  50 
under  50 

0     to    3 
3     to    7.5 
7.5  to  12.5 
12.5  to  25 
25     to  40 
35     to  50 
over  50 

14,600  to  14,900 
14,900  to  15,300 
15,300  to  15,600 
15,600  to  15,900 
15,800  to  14,800 
15,200  to  13,700 
13,700  to  11,000 

The  graphitic  anthracite  occurs  in  very  small  quantities 
and  mostly  in  Rhode  Island.  With  a  few  minor  exceptions 
the  anthracites  occur  only  in  Eastern  Pennsylvania  and  the 


FUELS  299 

semi-anthracites  are  almost  entirely  confined  to  the  western 
edge  of  this  field. 

The  semi-bituminous  coals  are  found  on  parts  of  the 
eastern  border  of  what  is  known  as  the  Appalachian  coal 
field,  extending  from  central  Pennsylvania  through  the 
intermediate  States  to  the  northern  part  of  Alabama.  The 
greater  part  of  this  enormous  bed  consists  of  eastern  bitu- 
minous coal.  Western  bituminous  coals  are  found  in  large 
beds  in  the  central  part  of  the  United  States,  principally  in 
the  States  of  Illinois,  Indiana  and  Kentucky  on  the  east  of 
the  Mississippi  River,  and  in  Iowa,  Kansas  and  Texas  to 
the  west  of  that  river. 

Lignite  is  found  in  small  quantities  in  nearly  all  of  the 
western  half  of  the  United  States  and  in  large  beds  in  the 
Dakotas,  Texas,  Arkansas,  Louisiana,  Mississippi  and 
Alabama. 

Peat  is  distributed  in  small  beds  throughout  practically 
all  of  the  United  States  and  is  continually  forming  in  many 
marshes  and  on  low-lying  lands. 

134.  Coal  Analyses.  Two  different  coal  analyses  are 
in  use,  the  simpler  being  known  as  the  proximate  analysis 
and  the  more  exhaustive  being  called  the  ultimate  analysis. 
Both  are  made  and  reported  on  a  weight  basis. 

The  proximate  analysis  assumes  coal  to  contain  four 
different  and  separable  things,  which  are  called  fixed  carbon, 
volatile  hydrocarbon  or  volatile  matter  or  volatile,  moisture 
and  ash. 

Moisture  is  determined  by  maintaining  a  small  quantity 
of  finely  ground  coal  at  a  temperature  of  about  220°  F. 
for  one  hour.  The  material  lost  during  this  time  is 
assumed  to  be  moisture  only  and  is  reported  as  such. 
Coal  from  which  the  moisture  has  been  driven  in  this  way 
is  called  dry  coal. 

Volatile  matter  is  determined  by  heating  a  sample 
from  which  the  moisture  has  been  driven,  or  a  fresh  sample. 
The  coal  is  maintained  at  a  red  to  white  heat  with  exclu- 


300  STEAM  POWER 

sion  of  air  until  there  is  no  further  loss  of  weight.  In 
the  case  of  a  previously  dried  sample  the  loss  under  these 
conditions  is  called  volatile  hydrocarbon.  If  the  sample 
was  not  previously  dried  a  separate  moisture  determina- 
tion is  made  on  a  similar  sample  and  the  weight  of  volatile 
is  found  by  difference. 

Fixed  carbon  is  found  by  combustion  of  a  sample  from 
which  the  moisture  and  volatile  have  been  driven,  the 
loss  under  these  conditions  being  assumed  to  be  entirely 
due  to  the  combustion  of  carbon. 

Ash  is  the  name  given  to  the  incombustible  material 
left  behind  after  determining  the  fixed  carbon. 

The  volatile  hydrocarbons  and  the  fixed  carbon  as 
determined  in  the  proximate  analysis  are  assumed  to  be 
the  only  combustible  parts  of  the  coal  and  their  sum  is 
called  the  combustible. 

Proximate  analyses  are  reported  in  three  different 
ways:  On  coal  as  received,  on  dry  coal,  and  on  combustible. 

Since  the  water  content  of  a  sample  of  coal  received 
at  any  plant  is  largely  a  matter  of  the  weather  conditions 
during  shipment,  the  best  idea  of  the  character  of  a  coal 
can  be  obtained  by  excluding  the  consideration  of  its 
moisture  content.  It  is  generally  best,  therefore,  to  convert 
analyses  to  a  dry  coal  basis,  that  is,  recalculate  the  per- 
centages of  volatile,  fixed  carbon  and  ash  on  the  assumption 
that  the  analysis  was  made  on  the  weight  of  coal  which  would 
result  from  drying  the  sample  that  was  actually  used.  Ex- 
cessive moisture  is,  however,  undesirable  for  steam-raising 
purposes,  and  the  amount  of  moisture  should  therefore  be 
determined  in  every  case. 

Ash  is  also  more  or  less  a  matter  of  accident  in  that  the 
amount  contained  is  largely  determined  by  the  care  used 
in  mining  and  subsequent  cleaning  of  the  coal.  While 
it  has  a  very  appreciable  effect  upon  the  character  of  the 
material  as  a  fuel  it  really  has  little  connection  with  the 
combustible  part  of  the  fuel.  For  purposes  of  classifica- 


FUELS  301 

tion,  therefore,  the  ash  should  also  be  eliminated  and  the 
analysis  given  on  the  basis  of  combustible. 

Sulphur  is  sometimes  reported  with  a  proximate  analy- 
sis. In  making  such  an  analysis  the  greater  part  of  the 
sulphur  is  really  driven  off  with,  and  regarded  as,  part  of 
the  volatile,  so  that  when  the  sulphur  content  is  desired  it 
must  be  determined  by  a  separate  analysis. 

The  ultimate  analysis  attempts  to  separate  the  dry 
combustible  into  the  various  elements  of  which  it  is  com- 
posed. The  percentages  of  carbon,  hydrogen,  oxygen, 
nitrogen  and  sulphur  are  determined  as  well  as  the  per- 
centage of  ash  in  dry  coal.  Such  analyses  show  the  carbon 
contents  of  coal  to  vary  from  about  98  per  cent  in  the 
graphitic  anthracite  through  about  97  per  cent  in  the 
semi-anthracite,  87  per  cent  in  semi-bituminous,  80  per  cent 
in  bituminous  and  74  per  cent  in  lignites  to  as  low  as  61 
per  cent  in  peats.  The  corresponding  figures  for  hydrogen 
run  from  about  1  per  cent  through  a  range  in  the  neigh- 
borhood of  5  per  cent  for  semi-bituminous  to  about  6  per 
cent  in  the  case  of  peat. 

Oxygen  varies  from  about  2  per  cent  or  less  in  the 
case  of  good  anthracite  to  as  high  as  33  per  cent  for  peat; 
nitrogen  generally  forms  about  1  per  cent  of  the  dry  fuel 
and  sulphur  from  1  to  3  per  cent. 

135.  Calorific  Value  of  Coals.  The  calorific  value  of 
coals  on  a  basis  of  combustible  has  been  shown  to  vary 
approximately  according  to  a  smooth  curve,  but  the  local 
variations  are  so  great  that  no  generally  applicable  formula 
for  calorific  value  has  yet  been  proposed.  The  formula 
commonly  used  is  based  upon  the  ultimate  analysis  and 
is  similar  to  that  suggested  as  approximately  applicable 
in  the  case  of  mixtures  of  combustibles.  It  is  known  as 
Dulong's  formula,  and  is 

f 62,000]  /        0\ 

B.t.u.  per  Ib.  =  14,600C  +       or       ( H-  ~ )  +4000S,     (100) 

1 52,000  J  N 


302  STEAM  POWER 

in  which  the  letters  refer  to  the  weight  of  the  various  ele- 
ments contained  in  one  pound  of  dry  coal. 

When  an  accurate  knowledge  of  the  calorific  value  of  a 
fuel  is  desired  it  should  be  obtained  by  means  of  a  fuel 
calorimeter.  There  are  many  varieties  of  this  instrument, 
but  practically  all  operate  on  the  same  general  principle. 
A  known  weight  of  fuel  is  completely  burned  within  a  vessel 
and  the  heat  liberated  is  absorbed  by  water  or  similar 
liquid.  From  measurements  of  liquid  temperatures  the 
heat  absorbed  by  the  liquid  can  be  determined,  and  this 
with  some  additions  for  losses  of  various  kinds  must  be  the 
heat  liberated  by  the  fuel. 

136.  Purchase  of  Coal  on  Analysis.     Until  quite  recently 
it  was  customary  to  buy  coal  from  the  lowest  bidder  pro- 
vided the  material  supplied  could  be  made  to  give  satis- 
factory results  in  the  plant.     Obviously  the  purchaser  knew 
nothing  regarding  his  purchase,  and  often  bought  quantities 
of  ash  and  moisture  at  the  price  of  combustible.     Now, 
however,  the  larger  power  plants  and  many  of  the  smaller 
are  buying  on  the  basis  of  analyses  and  calorific  values  as 
determined  in  calorimeters. 

A  certain  desirable  standard  analysis  is  set  and  cer- 
tain variations  are  allowed  from  it.  Wide  variations  are 
penalized  by  deducting  so  many  cents  per  ton  for  each 
variation  of  a  certain  degree,  and,  finally,  outside  limits 
are  set  for  moisture  and  ash  beyond  which  the  fuel  need  not 
be  accepted.  In  some  cases  limits  are  also  set  for  sulphur. 

This  is  the  logical  method  of  purchasing  coal  in  large 
quantities,  and  is  sure  to  come  into  very  general  use  as  its 
advantages  become  known. 

137.  Petroleum.     This  material  is  obtained  from  drilled 
wells  and  has  been  found  in  many  widely  separated  sections 
of  the  country.     The  oil  wells  of  Pennsylvania  and  neigh- 
boring States,  of  Oklahoma,  Texas  and  California  have  been 
the  most  productive  and  are  hence  the  most  widely  known. 

Natural  petroleum,  as  it  occurs  in  the  United  States,  is 


FUELS 


303 


generally  a  dark,  rather  thick,  oily  liquid  with  a  char- 
acteristic odor.  It  varies  widely  in  composition  so  far  as 
the  compounds  contained  are  concerned,  but  the  variations 
in  ultimate  composition,  specific  gravity  and  calorific  value 
are  comparatively  small. 

The  ultimate  analysis  of  crude  oil  generally  shows  about 
83  to  85  per  cent  of  carbon,  13  to  15  per  cent  of  hydrogen 
and  small  quantities  of  oxygen,  nitrogen  and  sulphur. 

The  specific  gravity  generally  lies  between  0.80  and  0.90 
and  in  most  cases  is  nearer  the  upper  figure.  It  is  common 
practice  to  express  the  gravity  in  terms  of  the  Beaume 
scale,  an  arbitrary  scale  developed  for  an  instrument  known 
as  the  Beaume  hydrometer.  This  device  is  arranged  to 
float  in  liquids  and  measures  the  gravity  by  the  distance  to 
which  it  sinks.  Various  corresponding  values  of  the  Beaume 
scale  and  specific  gravity  are  given  in  Table  XII  for  the 
region  most  used  in  connection  with  petroleum. 


TABLE  XII 
CORRESPONDING  BEAUME  READINGS  AND  SPECIFIC  GRAVITIES 


Beaum6  Reading. 

Specific  Gravity. 

Beaum6  Reading. 

Specific  Gravity. 

20 

0.9333 

34 

0.8536 

22 

0.9210 

36 

0.8433 

24 

0.9090 

38 

0.8333 

26 

0.8974 

40 

0.8235 

28 

0.8860 

42 

0.8139 

30 

0.8750 

44 

0.8045 

32 

0.8641 

46 

0.7954 

The  higher  calorific  value  varies  between  19,000  and 
20,000  B.t.u.  per  pound  and  the  lower  value  is  generally 
1000  to  1500  B.t.u.  lower. 

Crude  oil  is  sometimes  used  for  fuel,  but  this  is  unde- 
sirable, for  two  reasons.  First,  the  crude  oil  contains 
many  highly  volatile  constituents  which  can  be  distilled 


304  STEAM  POWER 

off  and  which  have  a  high  market  value  in  the  forms  of 
gasoline  and  allied  distillates.  Second,  the  presence  of 
these  highly  volatile  constituents  in  the  oil  makes  it  more 
dangerous,  as  combustible  vapors  are  given  off  in  large  quan- 
tities at  low  temperatures  and  the  mixtures  formed  with 
the- oxygen  of  the  air  are  often  highly  explosive. 

As  a  consequence,  the  material  generally  sold  as  fuel 
oil  is  a  residuum  left  after  distilling  off  the  more  volatile 
constituents  of  the  crude  oil.  It  has  practically  the  same 
properties  as  the  crude,  excepting  that  dangerous  vapors 
are  not  given  off  at  so  low  a  temperature. 

PROBLEMS 

1.  A  sample  of  coal  gives  the  following  proximate  analysis: 
moisture,    5%;     volatile,    4.25%;     fixed    carbon,    80.75%;     and 
ash,    10%.     Determine  the  percentage  of  combustible  and   the 
percentages  of  fixed  carbon  and  of  volatile  in  the  combustible. 

2.  What  variety  of  coal  is  indicated  by  the  values  obtained 
in  Prob.  1? 

3.  The  following  results  were  obtained  in  making  a  proximate 
analysis  of  a  sample  of  coal;   moisture,  7%;   fixed  carbon,  56.7%; 
volatile,   24.3%;    ash,   12%.     Determine  the  percentage  of  com- 
bustible and  the  percentages  of  fixed  carbon  and  of  volatile  in  the 
combustible.    What  variety  of  coal  is  indicated  by  these  values? 

4.  The  ultimate  analysis  of  a  sample  of  dry  coal  gave  the 
following  results:    carbon,   79.12%;    hydrogen,   4.14%;    oxygen, 
1.84%;    sulphur,  0.92%;  nitrogen,  0.74%;  ash,  13.24%.     Recal- 
culate these  values  for  an  ash-free  coal. 

5.  Determine  by  means  of  Dulong's  formula  the  upper  and 
lower  calorific  values  of  the  coal  described  in  Prob.  4. 

6.  The  ultimate  analysis  of  a  sample  of  crude  petroleum  from 
which  all  water  was  removed  gave  the  following  results:   carbon, 
85%;  hydrogen,  13%;  sulphur,  1.0%;   oxygen,  0.25%;   nitrogen, 
0.12%;  ash  (sand  and  similar  material),  0.63%.     Determine  the 
upper  and  lower  calorific  values  by  means  of  Dulong's  formula. 


CHAPTER  XVII 
STEAM   BOILERS 

138.  Definitions  and  Classification.  The  term  boiler 
is  generally  applied  to  the  combination  of  a  furnace  in  which 
fuel  may  be  burned  continuously  and  a  closed  vessel  in  which 
steam  is  generated  from  water  by  the  heat  liberated  within 
the  furnace. 

Boilers  are  classified  in  many  different  ways,  the  more 
important  being  given  in  the  following  schedule : 

CLASSIFICATION  OF  BOILERS 

(a)  Plain  cylindrical, 

(6)  Flue, 

(1)  According  to  form  /  \   «Ct    i 

(c)  Tubular, 

(d)  Sectional,  etc. 

(2)  According   to   location   off  (a)  Externally  fired,  and 

furnace  }  (6)  Internally  fired. 


(3)  According  to  use 


(a)  Stationary, 
(6)  Portable  (as  on  trucks, 
or  rollers), 

(c)  Locomotive, 

(d)  Marine. 


ff(a)  Horizontal, 

(4)  According  to  direction  of   ;  '  T     ,.      , 

.     .  .  { (6)  Inclined, 

principal  axis  ,  :  Tr    , .     , 

[(c)  Vertical. 

(5)  According  to  relative  posi-  f ,  x  ,Tr  ,      ,   , 

°  I  (a)  Water  tube, 

tions  of  water  and  hoU     '  ^.          , 

(b)  Fire  tube, 
gases 

305 


306 


STEAM  POWER 


Examples  of  boilers  of  the  different  types  mentioned  are 
given  in  subsequent  paragraphs. 

139.  Functions  of  Parts.  It  has  been  shown  that  there 
are  two  essentially  different  parts  in  the  apparatus  commonly 
known  as  a  steam  boiler,  the  furnace  and  the  boiling  vessel. 
A  simple  form  of  boiler  known  as  a  horizontal,  return  tubu- 
lar boiler,  or  an  H.R.T.  boiler,  is  shown  in  Figs.  181  and 
182  with  the  two  essential  parts  and  their  components 


Pressure  Regulator 


FIG.  181. — Sectional  Elevation  of  H.R.T.  Boiler  and  Furnace. 

indicated.  The  furnace  consists  essentially  of  the  combina- 
tion of  grates,  bridge  wall,  fire  and  ash  doors,  the  ash  pit 
and  the  space  above  the  grates.  It  is  the  function  of  the 
furnace  to  so  burn  the  fuel  that  the  maximum  amount 
of  heat  will  be  made  available  for  absorption  by  the  water 
within  the  boiling  vessel. 

It  is  the  function  of  the  boiling  vessel  to  transmit  to 
the  water  within  it  the  greatest  possible  quantity  of  the 
heat  thus  made  available  and  to  resist  successfully  the 


STEAM  BOILERS 


307 


tendency  to  rupture  under  the  action  of  the  high  internal 
pressure,  that  is,  the  pressure  of  the  steam. 

In  the  type  of  boiler  shown  the  fuel  is  "  fired  "  by 
hand,  that  is,  it  is 
spread  on  the  grate  by 
being  thrown  from  a 
scoop  shovel  through 
the  opened  fire  door. 
Air  enters  through  both 
doors  in  regulated  pro- 
portions and  in  such 
quantities  as  best  to 
approximate  complete 
combustion. 

The  hot  gases  re- 
sulting from  the  com- 
bustion pass  over  the 
bridge  wall,  along  the  FIG.  182. 

lower  part  of  the  boiler  Section  through  Furnace  of  H.R.T.  Boiler, 
shell  and  then  through 

the  fire  tubes,  or  flues,  toward  the  front  of  the  boiler  as 
shown  by  arrows  in  the  figure.  From  the  front  end  of  the 
tubes  the  products  of  combustion  pass  up  through  the 
smoke  box  to  "  breechings  "  or  "  flues,"  which  carry  them 
to  the  stack. 

Heat  is  received  by  the  water  within  the  vessel  in  two 
different  ways: 

(1)  The  hot  fuel  bed  on  the  grate  radiates  energy  in  the 
same  way  that  the  sun  or  any  other  glowing  body  radiates 
energy.     Some  of  this  energy  traverses  the  space  between 
fuel  bed  and  boiler  shell  and  ultimately  passes  through  that 
shell  to  the  water  within.     The  rest  of  the  radiated  energy 
passes  into  the  walls  surrounding  the  furnace  and  heats 
them  and  the  surrounding  atmosphere. 

(2)  The  hot  gases  of  combustion  pass  over  the  heating 
surface  of  the  boiler,  as  shown,  and  transmit  part  of  their 


308  STEAM  POWER 

heat  to  the  water  on  the  other  side  of  those  surfaces.  The 
rest  of  the  heat  which  they  carry  is  either  lost  to  the  surround- 
ing walls  or  is  carried  up  the  stack  by  the  gases  which 
leave  the  boiler  at  a  comparatively  high  temperature. 
This  temperature  ordinarily  ranges  from  about  500°  to  700° 
F.  and  in  extreme  cases  goes  even  higher. 

140.  Furnaces  and  Combustion.  In  most  forms  of 
boiler  the  water  within  the  boiler  has  practically  the  same 
temperature  as  the  steam  being  generated,  and  this  is 
generally  from  320°  to  400°  F.  Obviously  the  products 
of  combustion  cannot  be  cooled  by  the  water  to  a  tem- 
perature below  that  of  the  water,  so  that  the  gases  leaving 
the  boiler  in  an  ideal  case  would  have  a  comparatively 
high  temperature.  Practically,  it  is  found  undesirable  to 
attempt  to  reduce  the  temperature  of  the  gases  to  a  value 
even  approximating  that  of  the  water  and,  as  indicated 
above,  they  are  discharged  at  a  temperature  several  hundred 
degrees  higher.  In  order  that  the  maximum  amount  of 
heat  may  be  made  available  for  the  boiling  vessel  the  prod- 
ucts of  combustion  must  therefore  leave  the  furnace  with 
the  highest  possible  temperature,  and  the  ideal  furnace 
would  completely  burn  the  cheapest  fuel  available  in  such 
a  way  as  to  give  this  highest  possible  temperature  and  not 
to  generate  smoke. 

Real  furnaces  fall  far  short  of  this  ideal  performance, 
for  numerous  reasons.  The  more  important  of  these  are 
given  in  the  following  paiagraphs: 

(a)  Complete  Combustion  of  Carbon.  In  a  real  furnace 
the  combustion  of  the  carbon  of  the  fuel  may  be  incomplete 
in  two  senses;  first,  some  of  the  carbon  may  remain  entirely 
unoxidized  and  pass  off  with  the  ash,  and  second,  some  of 
the  carbon  may  be  burned  to  CO  instead  of  to  CO2. 

Imperfect  combustion  of  the  first  kind  can  result  from 
fuel  falling  through  the  openings  in  the  grate  before  it  has 
been  ignited  or  when  only  partly  burned,  or  it  can  result 
from  failure  to  get  air  to  some  of  the  carbon  in  sufficient 


STEAM  BOILERS  309 

quantities  to  burn  it  completely  before  all  of  the  surrounding 
fuel  has  been  converted  into  ash  and  the  locality  cooled 
down  to  such  an  extent  as  to  allow  the  unburned  carbon 
in  its  midst  to  cool  below  the  temperature  of  ignition. 

Imperfect  combustion  of  the  second  kind,  resulting 
in  the  formation  of  CO,  generally  results  either  from  a 
lack  of  sufficient  air  above  the  fuel  bed  or  from  an  excessive 
quantity  of  air  above  the  bed.  In  any  furnace  there  is  a 
tendency  toward  the  formation  of  CO  within  the  bed  of  fuel, 
and  the  deeper  the  bed  the  greater  this  tendency.  If  the 
CO  thus  formed  meets  sufficient  air  after  leaving  the  fuel, 
and  if  the  temperatures  of  CO  and  air  be  sufficiently  high, 
it  will  burn  to  CO2.  Part  of  the  air  for  what  may  be  called 
the  secondary  combustion  will  always  work  its  way  through 
the  fuel  bed,  because  it  is  impossible  to  bring  all  oxygen  in 
the  air  passing  through  into  contact  with  carbon  of  the  fuel. 
The  remainder  of  the  air  required  is  generally  admitted 
through  the  fire  door  and  is  heated  by  passing  over  the  front 
part  of  the  fuel  bed.  If  too  great  a  quantity  of  air  is  ad- 
mitted in  either  way  its  temperature  may  be  so  low  as  to 
cool  the  CO  below  its  temperature  of  ignition  and  thus  fail 
to  accomplish  tfie  object  sought. 

It  has  been  shown  that  the  combustion  of  pure  carbon 
with  the  theoretical  air  supply  would  give  gases  containing 
about  21  per  cent  by  volume  of  CO2.  If  the  combustible  of 
real  coal  be  assumed  to  consist  entirely  of  carbon,  the  same 
proportion  of  CO2  would  result  from  ideal  combustion. 
Practically,  it  is  so  difficult  to  bring  the  oxygen  of  the  air 
into  contact  with  the  carbon  of  the  fuel  that  a  large  excess 
is  always  used,  the  excess  coefficient  ranging  from  about 
1.3  to  over  2  and  averaging  about  1.5  to  1.7  under  very 
good  conditions.  The  latter  figures  correspond  to  percent- 
ages of  CO2  of  14  and  12  respectively,  but  a  value  as  low 
as  10,  which  corresponds  roughly  to  an  excess  coefficient 
of  about  2,  is  not  at  all  uncommon  and  is  generally  regarded 
as  a  very  good  result  except  in  the  largest  plants. 


310  STEAM  POWER 

(b)  Complete  Combustion  cf  Hydrocarbons.  The  hydro- 
carbons which  appear  as  volatile  matter  in  the  proximate 
analysis  are  practically  all  distilled  from  the  fuel,  as  it  is 
heated  in  the  furnace  before  ignition  in  the  same  way  as 
when  making  a  proximate  analysis.  If  they  are  to  be 
completely  burned  they  must  be  mixed  with  the  requisite 
quantity  of  air  after  distillation  and  both  the  vapors  and  the 
air  must  be  maintained  at  a  sufficiently  high  temperature 
until  combustion  is  complete.  Part  of  the  air  for  the  com- 
bustion of  distilled  volatile  filters  through  the  fuel  bed 
and  the  rest  must  be  admitted  through  the  fire  door  or  in 
some  equivalent  manner. 

If  the  flame  formed  by  burning  hydrocarbons  is  allowed 
to  come  in  contact  with  cold  surfaces,  as,  for  instance, 
the  heating  surfaces  of  the  boiler,  the  gases  are  cooled 
below  the  temperature  of  ignition  and  combustion  ceases. 
This  results  in  the  deposit  of  soot  (unburned  carbon)  upon 
the  heating  surfaces  of  the  boiler  and  in  the  carrying  of 
soot  and  unburned  hydrocarbons  up  the  stack.  The  soot 
and  some  of  these  hydrocarbons  form  the  unsightly  smoke 
so  familiarly  associated  with  some  stacks. 

Or,  if  the  air  supply  is  at  a  sufficiently  high  temperature, 
but  is  insufficient  in  quantity,  the  hydrocarbons  are  in- 
completely burned  and  smoke  results. 

The  formation  of  smoke  can  be  conveniently  studied 
by  means  of  the  ordinary  kerosene  lamp.  Such  a  lamp 
operates  by  burning  hydrocarbons  of  the  same  general 
character  as  those  distilled  from  solid  fuels.  The  hydro- 
carbons are  drawn  up  by  the  wick  in  the  form  of  liquids, 
are  vaporized  by  heat  near  the  top  of  the  wick  and  then 
combine  with  oxygen  from  the  atmosphere  to  give  the 
luminous  kerosene  flame. 

If  the  flow  of  kerosene  and  the  air  supply  are  properly 
adjusted  and  if  the  temperature  is  high  enough,  the  com- 
bustion results  in  the  formation  of  invisible  and  practically 
odorless  gases.  If,  however,  the  air  supply  be  decreased 


STEAM  BOILERS  311 

or  be  greatly  cooled,  a  very  smoky  and  very  odorous  combus- 
tion ensues.  The  same  result  could  be  obtained  by  the  use 
of  too  great  a  quantity  of  air,  a  condition  often  attained 
when  the  supply  of  kerosene  in  the  bowl  of  the  lamp  is  almost 
exhausted. 

The  effect  of  a  cold  surface  is  easily  seen  by  inserting 
a  cold  metallic  or  porcelain  surface  into  the  tip  of  the  flame 
and  then  withdrawing  it.  It  will  be  found  covered  with 
soot. 

(c)  Advantages  and  Disadvantages  cf  Excess  Air.  It 
has  been  shown  that  excess  air  is  practically  necessary  in 
the  real  furnace  in  order  to  insure  against  a  deficiency  at 
any  point,  and  it  is  thus  advantageous  in  that  it  makes  the 
combustion  more  nearly  complete  than  would  otherwise 
be  the  case.  On  the  other  hand,  excess  air  represents  just 
so  much  excess  material  to  be  heated  at  the  expense  of  heat 
liberated  by  combustion  and  hence  decreases  the  maximum 
temperature  attained.  A  sufficiently  great  supply  of  excess 
air  could  so  reduce  the  temperature  that  even  if  combus- 
tion were  complete  very  little  heat  would  be  made  available 
for  absorption  by  the  boiling  vessel,  because  the  temperature 
attained  by  the  products  of  combustion  would  be  too  low. 

Excess  air  in  large  quantities  may  also  result  in  cooling 
unburned  gases  before  combustion  to  such  an  extent  as  to 
make  the  completion  of  combustion  impossible. 

141.  Hand  Firing.  The  commonest  type  cf  furnace  is 
that  shown  in  Figs.  181  and  182,  and  the  commonest  method 
of  hand  firing  consists  in  spreading  a  layer  of  fuel  as  evenly 
as  possible  over  the  entire  surface  of  the  fuel  bed  as  often 
as  required  to  replace  the  fuel  burned  away.  At  such  inter- 
vals as  experience  shows  to  be  necessary  the  fire  is  cleaned, 
that  is,  the  ashes  are  worked  out  from  under  the  fuel  by 
means  of  slice  bars,  so  that  practically  nothing  but  live 
fuel  resting  on  a  thin  layer  of  ash  remains  behind. 

This  method  is  open  to  many  serious  objections;  the  more 
important  are: 


312  STEAM  POWER 

1.  There  is  a  gradual  increase  in  thickness  of  fuel  bed 
from  the  time  of  one  cleaning  until  the  time  of  the  next. 
This  gives  a  constantly  changing  set  of  requirements  for 
the  proper  proportions  of  air  entering  below  and  above 
the  fuel  bed  and  a  constantly  changing  resistance  to  flow 
of  air  through  the  bed,  so  that  great  skill  is  necessary  if  the 
best  conditions  are  to  be  maintained  throughout. 

2.  There  is  always  a  tendency  for  a  fuel  bed  to  burn 
faster  at  some  points  than  at  others,  due  to  the  accidental 
distribution   of   fuel,    ash    and    air.     Where    "  holes  "    are 
formed  in  this  way  large  quantities  of  comparatively  cold 
air  can  pass  through  with  the  consequences  already  enumer- 
ated.    It  takes  considerable  skill  and  watchfulness  on  the 
part  of  the  fireman  to  prevent  the  formation  and  continued 
existence  of  such  holes. 

3.  The   firing   door  must   be   opened   wide  every  time 
that  fuel  is  to  be  fired,  that  is,  at  intervals  varying  from  two 
or  three  minutes  to  fifteen  or  more,  depending  on  load, 
character  of  fuel,  etc.     While  the  door  is  open  large  quanti- 
ties of  cold  air  readily  flow  into  the  furnace  and  cool  down 
all  parts  of  it,  and  a  proportionately  smaller  amount  will 
ordinarily  pass  through  the  fuel  bed.     The  result  of  this  on 
the  flue  gases  and  operation  of  the  boilers  has  already  been 
considered,  but  there  is  another  result  of  equal  or  greater 
importance.     As  a  consequence  of  this  action  the  volatile 
hydrocarbons  distilled  off  from  the  freshly  fired  fuel,  which 
are  themselves  at  a  comparatively  low  temperature,   are 
surrounded  on  all  sides  by  cooled  walls  and  come  in  contact 
with  cold  air  only.     The  chances  of  their  burning  completely 
are  very  slight,  and  a  great  part  of  these  volatilized  materials 
passes  off  unburned  as  invisible  gas  and  as  smoke.     Ob- 
viously the  greater  the  volatile  content  the  greater  the  dif- 
ficulty, so  that  anthracite  causes  least  trouble  in  this  way, 
while  most  bituminous  coals  give  heavy  black  smoke  when 
burned  under  these  conditions. 

The  cooling  down  of  the  interior  of  the  furnace  during 


STEAM  BOILERS  313 

firing  is  accompanied  by  the  covering  of  the  fuel  bed  with 
cold  fuel,  so  that,  for  the  time  being,  very  little  radiant 
heat  enters  the  boiling  vessel,  and  the  gases  which  come  in 
contact  with  its  surface  are  comparatively  cool.  The 
maintenance  of  a  constant  steam  pressure  under  these  con- 
ditions is  practically  impossible,  but  the  difficulties  can  be 
partly  overcome  by  very  frequent  firing  of  small  quantities, 
so  that  the  door  is  open  a  very  short  time  and  also  that  the 
layer  of  fuel  is  very  thin  and  does  not  cut  off  much  heat. 

4.  The  cleaning  of  the  fire  necessitates  keeping  the 
fire  door  open  for  several  minutes,  with  results  of  the  same 
variety  as -those  just  enumerated. 

Summing  up  these  difficulties,  they  divide  themselves 
into  two  classes — those  which  can  be  almost  or  entirely 
eliminated  by  skill  of  a  very  high  order  and  those  which  are 
inherent  and  cannot  be  eliminated  by  skill.  It  will  also 
be  observed  that  all  should  give  more  trouble  with  fuels 
high  in  volatile  than  with  those  of  the  anthracite  variety, 
both  as  to  incomplete  combustion  and  to  the  formation 
of  smoke. 

Several  other  methods  of  hand  firing  have  been  proposed, 
particularly  for  use  with  bituminous  coals,  and  some  of 
them  have  been  successfully  utilized  in  isolated  instances. 
Nearly  all  depend  upon  covering  only  part  of  the  fuel  bed 
at  one  time  and,  by  alternating  the  parts  covered  in  this 
way,  fresh  fuel  on  one  part  of  the  bed  is  coked  while  air  is 
heated  by  coming  in  contact  with  the  uncovered  incandescent 
part  of  the  bed  and  is  therefore  in  proper  condition  to  burn 
more  perfectly  the  volume  of  hydrocarbons  being  distilled 
off.  These  methods  are  all  good,  but  they  involve  a  great 
deal  of  careful  work  and  a  high  degree  of  skill  on  the  part  of 
the  fireman. 

Other  methods  of  eliminating  some  of  the  difficulties 
depend  upon  modifications  of  the  furnace  and  air  supply. 
Most  attempt  to  entirely  surround  the  fuel  and  the  gases 
given  off  with  heavy  masses  of  brick  work  and  tile,  so  that 


314 


STEAM  POWEE 


enough  heat  will  be  stored  during  incandescent  periods 
to  tide  over  the  periods  of  cooling.  Some  forms  have 
combined  with  this  idea  a  series  of  air  ducts  in  the  brick 
work  so  arranged  that  air  on  its  way  to  the  furnace  passes 
through  these  ducts  and  is  heated.  In  some  cases  the  air 
supply  is  automatically  controlled  and  more  air  is  supplied 
above  the  fire  during  the  period  of  distillation,  or  coking, 
as  it  is  called,  than  during  the  following  period,  when  the 
coked  coal  is  brightly  incandescent  and  little  volatile 
matter  is  present. 


MM          -3yAT.CuVy.v\v-  -f^a^' .  -.O- 


'uiMfiUi' 


pra3S»Wi^:t 
If  WS 


ewm. 


FIG.  183. 


In  some  hand-fired  furnaces  which  are  intended  for  use 
with  bituminous  coals  that  give  a  long  flame  the  parts  of  the 
boiling  vessel  within  range  of  the  flames  are  covered  with 
tiles.  This  prevents  impingement  of  unburned  gases  upon 
cool  surfaces  and  thus  tends  to  prevent  the  formation  of 
smoke  and  incomplete  combustion. 

Carrying  this  principle  to  its  logical  conclusion  results 
in  the  installation  of  the  grate  in  a  firebrick  chamber  in 
front  of  the  boiler-setting  proper,  as  shown  in  Fig.  183. 
Such  a  device  is  known  as  a  Dutch  oven  and  is  often  very 
efficient  in  totally  or  partially  preventing  the  formation  of 


STEAM  BOILERS  315 

smoke.  It  does  not,  however,  give  as  high  an  economy 
as  might  be  expected,  because  a  great  part  of  the  radiant 
heat  of  the  fire  does  not  reach  the  boiler  surfaces  and  because 
the  large  external  surface  results  in  great  radiation  losses 
to  atmosphere. 

Another  interesting  modification  consists  of  reversing 
the  direction  of  the  draft,  that  is,  the  direction  in  which  the 
air  passes  through  the  fuel  bed.  The  type  of  furnace  al- 
ready described  is  known  as  an  updraft  furnace,  because  the 
air  passes  upward  in  flowing  through  the  bed.  The  modi- 
fied type  here  referred  to  is  called  a  downdraft  furnace, 
because  the  air  flows  downward  in  passing  through  the  fuel. 

In  downdraft  furnaces  the  coal  is  fired  on  top  of  the 
grate  as  in  other  types,  but  the  air  is  admitted  above,  flows 
downward  toward  what  would  normally  be  the  ashpit, 
and  from  there  on  over  the  heating  surfaces  of  the  boiler. 
Fresh  coal  fired  on  top  of  the  incandescent  bed  in  such  a 
furnace  distills  as  in  other  types,  but  the  volatiles  are  mixed 
with  the  entering  air  and  are  carried  downward  through  the 
hot  bed  so  that  ideal  conditions  for  combustion  are  more 
nearly  attained.  In  some  forms  there  is  a  second  updraft 
grate  beneath  the  downdraft  grate.  This  second  grate 
receives  partly  burned  coals  falling  through  from  the  upper 
grate  arid  holds  them  until  combustion  is  practically  com- 
pleted. 

In  downdraft  furnaces  the  grate  bars  are  generally  made 
of  pipes,  and  water,  from  the  boiler  or  on  its  way  to  the 
boiler,  is  circulated  through  them.  If  this  were  not  done 
the  grates  would  quickly  warp  out  of  shape  and  ultimately 
burn  away  because  of  the  high  temperatures  to  which  they 
are  subjected. 

142.  Mechanical  Grates.  In  order  to  overcome  the  dif- 
ficulties arising  from  opening  the  doors  for  the  purposes  of 
cleaning  the  fire,  numerous  so-called  rocking,  shaking, 
self -cleaning,  or  dumping  grates  have  been  developed. 
These  are  generally  built  up  of  grate  bars  which  have  a 


316 


STEAM  POWER 


rough  T  or  an  inverted  L  section  with  the  upper  horizontal 
branch  of  the  T  or  inverted  L  slightly  rounded,  as  shown  in 
Fig.  184.  These  bars  are  arranged  in  groups  with  their 
longitudinal  axes  running  across  the  grate,  and  they  are  so 
supported  that  they  can  be  rocked  about  a  point  in  the  verti- 
cal leg  of  the  T  or  L  by  means  of  levers  located  at  the  front 
of  the  boiler.  By  rocking  the  bars  the  lower  part  of  the  fuel 
bed  which  has  been  burned  to  ash  can  be  dropped  into  the 
ash  pit,  while  the  upper  part  is  sufficiently  agitated  to  close 
up  holes  which  may  have  formed,  and  this  can  all  be  done 


FIG.  184. 


with  the  doors  closed.  Or,  if  desired,  part  or  all  of  the  fuel 
bed  can  be  dropped  into  the  ash  pit  by  a  similar  rocking 
motion. 

143.  Smoke  and  Its  Prevention.  An  idea  of  the  reasons 
for  the  formation  of  smoke  will  have  been  obtained  from  the 
preceding  paragraphs.  A  reasonably  skillful  fireman  should 
have  little  difficulty  in  burning  anthracite  coals  in  the  simpler 
forms  of  furnaces  without  smoke,  but  it  is  almost  impossible 
to  commercially  burn  many  of  the  varieties  of  bituminous 
coals  in  this  way  without  the  formation  of  excessive  volumes 
of  dense  black  smoke  at  intervals  immediately  following- 
each  firing. 


STEAM  BOILERS  317 

Aside  from  all  aesthetic  and  sanitary  considerations, 
smoke  is  undesirable  because  it  represents  poor  furnace 
conditions  and  waste.  The  actual  loss  of  carbon  in  visible 
smoke  is  generally  almost  negligible  in  comparison  with 
the  other  losses  in  the  form  of  unburned  hydrocarbons, 
the  lowered  initial  temperature,  etc.  All  of  these  losses 
combined  represent  a  waste  of  considerable  magnitude. 

The  proper  method  of  smoke  elimination  is  not  the 
combustion  or  removal  of  smoke  already  formed,  but  it 
is  the  burning  of  fuels  in  such  ways  as  not  to  form  any 
appreciable  quantity  in  the  first  place.  To  accomplish 
this  end  the  following  must  be  achieved: 

1.  Coal    must    be    fired    continuously   and    uniformly 
without  the. opening  of  doors  which  admit  cold  air  to  the 
furnace. 

2.  Volatiles   must   be   distilled    continuously   and   uni- 
formly and  in  such  a  place  that  they  are  given  ample  oppor- 
tunity to  mix  with  proper  proportions  of  air  and  to  bum 
completely  before  coming  in  contact  with  cool  surfaces. 

3.  The    air   supply   must   be    properly    controlled   and 
tempered  to  meet  the  demands  of  the  fuel  both  in  and 
above  the  bed. 

4.  The    fire    bed    must    be    worked    continuously    and 
uniformly  so  as  to  eliminate  ashes  as  rapidly  as  formed 
and  to  maintain  a  bed  of  uniform  depth  and  condition. 

Some  of  these  necessary  conditions  can  be  attained  by 
the  use  of  the  various  forms  of  hand-fired  furnaces  already 
described  but,  even  in  the  hands  of  skillful  and  industrious 
men,  it  is  impossible  to  meet  all  of  them.  Mechanical 
stokers  which  more  nearly  approach  the  ideals  set  have 
therefore  been  developed  and  are  widely  used. 

144.  Mechanical  Stokers.  These  mechanical  devices  are 
useful  for  two  reasons — they  eliminate  a  great  deal  of  labor 
and  they  make  possible  the  burning  of  many  varieties  of 
refractory  fuels  without  the  formation  of  excessive  quanti- 
ties of  smoke. 


318  STEAM  POWER 

Despite  the  good  results  which  can  be  achieved  by 
their  use,  mechanical  stokers  are  not  installed  in  small 
plants  as  often  as  might  be  expected.  This  is  because 
good  stokers  are  very  expensive  in  comparison  with  hand- 
fired  furnaces  and,  despite  economy  of  fuel,  do  not  generally 
show  a  financial  saving  unless  their  use  eliminates  the 
services  of  several  firemen. 

It  is  generally  assumed  that  one  man  can  care  for  water, 
coal  and  ashes  for  about  200  boiler  horse-power  or  can 
handle  coal  only  for  about  500  boiler  horse-power.  Expe- 
rience has  shown  that  one  man  can  care  for  about  2000  to 
5000  boiler  horse-power  when  the  boilers  are  equipped  with 
good  stokers  and  coal-handling  apparatus. 

Financial  calculations  will  generally  show  stokers  to 
be  profitable  investments  for  plants  of  2000  or  more  boiler 
horse-power.  Where  they  are  installed  in  smaller  plants, 
the  absolute  necessity  of  eliminating  smoke  or  the  use  of 
very  poor  varieties  of  coal  have  generally  dictated  their  use. 

Mechanical  stokers  can  be  roughly  divided  into  two 
types,  those  which  duplicate  hand  spreading  of  fuel  and 
are  known  as  sprinkler  stokers,  and  those  which  supply 
fuel  at  one  or  more  points  and  work  it  progressively  toward 
the  ash  end  of  the  apparatus  as  it  burns.  The  first  type 
has  not  been  widely  installed,  though  it  is  possible  that 
it  may  meet  with  more  popular  approval  after  further 
development. 

Stokers  of  the  second  type  may  be  roughly  divided 
into  five  classes,  which  are 

1.  Chain  grates. 

2.  Inclined  stokers. 

3.  Underfeed  stokers. 

4.  Combinations  of  above. 

5.  Powdered  coal  stokers. 

A  chain  grate,  as  made  by  the  Illinois  Stoker  Company, 
is  illustrated  in  Figs.  185,  186,  187,  and  188.  It  consists 


STEAM  BOILERS 


319 


320 


STEAM  POWER 


of  a  broad  chain  made  up  of  a  great  number  of  small  links 
and  carried  on  toothed  wheels  and  roller  wheels  supported 
in  a  frame  which  can  be  wheeled  into  position  within  the 


FIG.  186.— Sprocket  and  Links  of  Illinois  Chain  Grate. 

mm 


TOP  VIEW  OF  CHAIN 

SHOWING  DISTRIBUTION  OF  AIR  SPACES 


BOTTOM  VIEW  OF  CHAIN 

SHOWING  ROLLERS  FOR  DRIVING-SPROCKET 
ENGAGEMENT 


FIG.  187. 

boiler  setting.  The  general  arrangement  of  the  chain  and 
rollers  is  shown  in  Fig.  185;  details  of  the  front  or  driving 
rollers  and  of  the  links  are  shown  in  Fig.  186;  a  top  and  bot- 
tom view  of  part  of  the  chain  is  given  in  Fig.  187;  and 
Fig.  188  is  a  perspective  view  of  the  frame  showing  the 


STEAM  BOILERS  321 

tracks  on  which  it  may  be  rolled  into  and  out  of  the  boiler 
setting. 

The  chain  is  driven  slowly  in  the  direction  indicated 
by  the  arrows  in  Fig.  185  by  power  applied,  through  worm 
gearing,  to  the  shaft  of  the  toothed  wheels  at  the  front 
of  the  stoker.  Coal  feeds  automatically  from  the  hopper 
by  gravity  and  is  carried  into  the  combustion  space  by 
the  moving  chain,  the  thickness  of  the  bed  being  controlled 


FIG.  188. — Framework  of  Illinois  Chain  Grate. 

by  the  height  of  the  adjustable  gate  shown.  As  the  fuel 
enters  the  furnace  it  passes  under  the  coking  arch,  which 
spans  the  entire  front  part  of  the  grate  and  which  is  main- 
tained at  a  high  temperature  by  heat  radiated  from  the  in- 
candescent fuel  nearer  the  inner  end  of  the  grate.  The 
volatiles  are  distilled  from  the  fresh  coal  by  heat  received 
from  this  arch  and  are  heated  and  mixed  with  air  at  this 
point.  The  coked  fuel  is  then  carried  on  into  the  furnace 
and  burned,  the  refuse  being  discharged  at  the  bridge  wall. 
If  the  thickness  of  bed  and  speed  of  chain  travel  are 
properly  adjusted,  all  of  the  fuel  can  be  coked  before  pass- 
ing out  from  under  the  arch  and  can  be  burned  almost 


322 


STEAM  POWER 


completely  before  reaching  the  bridge  wall,  so   that  prac- 
tically ashes  only  will  be  discharged. 


The  apron  shown  at  A  in  Fig.  185  is  used  to  prevent 
the  free  passage  of  air  to  the  part  of  the  chain  carrying 


STEAM  BOILERS 


323 


practically  nothing  but  ash,  as  this  would  result  in  excessive 
dilution  of  the  products  of  combustion. 

A  stoker  of  this  type  installed  under  a  horizontal  return- 
tubular  boiler  is  shown  in  Fig.  189.  In  the  illustration  part 
of  the  side  frame  of  the  stoker  is  broken  away  in  order  to 
show  the  chain  and  its  roller  guides.  The  eccentric  shown 


FIG.  190. — Details  of  Feed  Mechanism,  F.onsy  Stoker. 

near  the  top  of  the  front  of  the  boiler  drives  the  chain 
through  an  arm  of  adjustable  length,  which  makes  possible 
the  control  of  the  speed  of  chain  travel. 

An  inclined  stoker  with  front  feed  and  a  step  grate, 
known  as  the  Honey  stoker,  is  shown  in  Figs.  190  and  191. 
The  fuel  is  fed  out  of  the  hopper  and  onto  the  dead  plate 
by  means  of  the  reciprocating  pusher.  From  the  dead 
plate  it  is  pushed  down  upon  the  grate  bars  by  the  follow- 
ing fuel.  These  bars  are  rocked  mechanically  so  that  their 


324 


STEAM  POWER 


STEAM  BOILERS 


325 


tops  alternately  assume  horizontal  and  inclined  positions, 
and  this  action  feeds  the  fuel  downward  until  it  is  dis- 
charged onto  the  dumping  grate.  The  material  collect- 
ing on  this  grate  is  periodically  dropped  by  hand  into  the 
ashpit. 

The  fuel  is  coked  while  passing  under  the  coking  arch 


FIG.  192. — Transverse  Section  of  the  Murphy  Stoker. 

and  the  coked  material  is  practically  completely  burned 
by  .the  time  it  has  traveled  down  the  grate.  The  volatiles 
are  mixed  under  the  coking  arch  with  heated  air  which  has 
passed  through  the  grate  and  with  heated  air  forced  in 
above  the  fuel  by  steam  jets. 

An  inclined  stoker  of  the  side-feed  type  with  bar  grates, 
known  as  the  Murphy  stoker,  is  illustrated  in  Figs.  192, 
193  and  194.  This  stoker  is  provided  with  two  coal- 


326 


STEAM  POWER 


magazines  or  hoppers  which  are  placed  horizontally  in 
the  side  walls  of  the  boiler  setting  and  feed  fuel  onto  the 
inclined  grate  bars,  Fig.  192,  which  carry  it  downward 
toward  the  lower  point  of  the  V  formed  by  the  grates. 
The  grate  bars,  Fig.  194,  are  alternately  fixed  and  mov- 
able, the  movable  bars  being  hung  from  above  and  their 


%%pilfe^^ 

r^^'^/fU^?:^:^^::;^ 

FIG.  195.— Longitudinal  Section,  Murphy  Stoker. 

lower  ends  being  moved  up  and  down  by  power  furnished 
by  a  small  steam  engine. 

A  toothed  bar  arranged  for  rotation  by  hand  or  by  power 
is  located  at  the  bottom  of  the  V  and  is  used  for  grinding 
up  ash  and  clinker  which  is  too  large  to  fall  through  into 
the  ash  pit.  This  bar  is  kept  cool  by  making  it  hollow 


STEAM  BOILERS 


327 


and  connecting  one  end  to  the  smoke  flues  or  stack  so 
that  air  is  constantly  drawn  through  it. 

The  location  of  the  coking  arch  and  the  method  used 
j"or  supplying  warm  air  should  be  evident  from  the  figures. 

A  stoker  of  this  variety  is  shown  in  place  under  a  hori- 
zontal water-tube  boiler  in  Fig.  195. 

An  underfeed  stoker  made  by  the  American  Stoker  Com- 


Movable 
Grate  Bar 


FIG.  194.— Grate  Bars  of  Murphy  Stoker. 

pany  is  shown  in  Figs.  166  and  197.  Coal  is  fed  from  the 
hopper  onto  the  reciprocating  bottom  B  by  means  of  the 
reciprocating  pusher  P.  The  latter  forms  the  bottom  of 
a  trough  as  shown  in  Fig.  197,  and  its  reciprocating  motion 
feeds  the  coal  upward  and  out  of  this  trough  so  that  it 
spills  over  onto  the  inclined  grate  bars.  The  reciprocating 
motions  are  all  obtained  from  the  direct-acting  steam  cylin- 
der shown. 

The  inclined  grate  bars  are  alternately  fixed  and  mov- 


328 


STEAM  POWER 


able,  the  movable  bars  sliding  back  and  forth  at  right  angles 
to    the    trough  .under    the    action    of    horizontal     rocking 


t 

I 


bars  7?.  This  action  gradually  feeds  the  fuel  downward 
and  toward  the  side  of  the  furnace,  the  refuse  finally  land- 
ing on  the  dumping  trays  shown. 


STEAM  POWER 


329 


FIG.  196. — Longitudinal  Section  of  Class  E  American  Stoker. 


FIG.  197. — Cross  Section  of  Class  E,  American  Stoker. 


330 


STEAM  BOILERS 


STEAM  BOILERS 


331 


Air  enters  the  duct  below  the  trough  through  the 
adjustable  gate  G,  controlled  by  crank  C,  and  part  of  it 
passes  out  through  holes  H  near  the  top  of  the  trough 
Fig.  197.  The  remainder  passes  down  through  the  hollow 


FIG.  199.— Taylor  Stoker  Under  Horizontal  Water-tube  Boiler. 


grate  bars  and  into  the   heated  air  box  from  which  it  flows 
upward  between  the  grate  bars. 

It  will  be  observed  that  the  coal  is  fed  onto  the  grate 
from  below,  so  that  all  volatiles  distilled  off  must  pass  up- 
ward through  the  incandescent  fuel  before  entering  the 
space  above  the  fuel  bed.  Part  of  the  air  which  is  to  burn 


332  STEAM  POWER 

this  volatile  matter  also  passes  through  the  fuel  bed  and 
the  remainder  flows  over  the  incandescent  fuel  from  the 
opening  shown  near  the  hopper  in  Fig.  196.  The  air  and 
volatiles  are  thus  raised  to  a  high  temperature  and  well 
mixed,  and  the  operation  is  continuous  and  uniform,  all 
tending  to  facilitate  smokeless  combustion. 

Another  variety  of  underfeed  stoker  known  as  the 
Taylor  stoker  is  shown  in  Fig.  198  (a),  (6)  and  (c). 
This  stoker  is  built  up  of  alternated  retorts  and  air 
boxes,  the  proper  number  to  give  the  desired  width 
of  stoker  being  used.  Coal  is  fed  from  the  hoppers 
into  the  retorts  by  the  upper  ram  or  plunger  shown 
in  Fig.  198  (b)  and  part  of  it  is  again  pushed  forward  by 
the  lower  ram  or  plunger.  The  stroke  of  the  lower 
plunger  can  be  regulated  and  in  this  way  the  relative 
quantities  of  coal  pushed  forward  in  the  upper  and  lower 
parts  of  the  retorts  can  be  controlled.  The  coal  spreads 
over  the  tuyere  blocks  which  form  the  inclined  tops  of 
the  air  boxes  and  forms  a  comparatively  even,  inclined 
layer  of  fuel. 

Coking  proceeds  under  the  incandescent  fuel  which 
forms  the  upper  surface  of  this  layer,  and  the  volatiles 
mix  with  air  entering  through  the  hot  tuyeres  and  pass 
upward  through  the  hot  fuel  above. 

In  this  stoker  advantage  is  often  taken  of  the  fact  that 
the  draft  (pressure  of  air)  required  with  underfeed  stokers 
is  so  great  that  it  can  be  more  economically  attained  by  the 
use  of  a  fan  than  by  the  use  of  a  stack.  The  fan  and  the 
coal-feeding  plungers  are  both  connected  to  one  engine 
and  the  speed  of  this  engine  is  automatically  controlled  by 
the  steam  pressure  within  the  boiler.  As  this  pressure 
decreases  the  engine  speeds  up,  thus  delivering  more  coal 
and  air  and  as  the  pressure  increases  the  engine  slows  down 
with  opposite  results.  By  properly  fixing  the  travel  of  the 
plungers  initially,  the  best  relative  proportions  of  air  and 
coal  are  set  for  the  entire  range  of  loads  to  be  carried  and  the 


STEAM  BOILERS  333 

variation  of  both  is  thereafter  in  approximately  the  same 
proportions. 

A  stoker  of  this  type  in  position  under  a  horizontal 
water-tube  boiler  is  shown  in  Fig.  199.  A  double-ended 
arrangement  of  Taylor  stokers  as  used  under  very  large 
water-tube  boilers  is  shown  in  Fig.  200. 

Powdered-coal  stokers  have  been  invented  in  great 
number  and  are  successfully  used  in  several  of  the  indus- 
tries. They  have  not,  however,  "been  extensively  used 
under  steam  boilers,  although  isolated  installations  have 
been  reported  as  giving  satisfactory  results. 

In  all  cases,  bituminous  coal  is  crushed  to  a  fine  powder 
and  injected  into  the  furnace  with  the  necessary  air  for  com- 
bustion, the  air  under  pressure  generally  being  made  to 
mechanically  entrain  the  coal  dust  and  carry  it  into  the  fur- 
nace. The  mixture  of  fine  coal  and  air  gives  an  intensely 
hot  blow-pipe  type  of  flame,  and  firebrick  and  tile  are 
generally  used  to  prevent  it  from  impinging  directly  upon 
metallic  parts. 

Oil  firing  is  essentially  a  mechanical,  rather  than  a  manual 
process,  and  while  oil  burners  are  not  ordinarily  under- 
stood as  belonging  to  the  class  of  mechanical  stokers,  they 
have  all  the  essential  characteristics  of  such  apparatus. 

To  burn  oil  successfully  under  a  boiler  it  must  be  finely 
atomized  and  mixed  with  the  necessary  quantity  of  air, 
and  there  must  be  sufficient  open  space  within  the  furnace 
for  the  free  development  of  the  flame  and  the  completion 
of  combustion  before  impingement  on  cool  surfaces. 

Oil-burning  furnaces  are  generally  given  a  rather  large 
volume;  considerable  firebrick  is  used  in  such  ways  as  to 
give  incandescent  walls  and  baffles  to  assist  ignition  and 
combustion,  and  all  heating  surfaces  are  arranged  so  that 
they  are  not  in  the  direct  path  of  the  flame. 

The  atomization  of  the  oil  is  effected  in  two  distinctly 
different  ways.  In  some  forms  of  burners  it  is  brought 
about  by  mechanical  means,  the  oil  being  pumped  through 


334 


STEAM  POWER 


FIG.  200. — Double-ended  Arrangement  of  Taylor  Stoker  under  Sterling 
Type  W.  Boiler. 


STEAM  BOILERS  335 

a  nozzle  of  some  sort  which  is  so  shaped  that  the  issuing 
jet  breaks  up  into  a  great  number  of  very  small  particles. 
In  other  forms,  steam  is  used  to  break  up  the  jet,  the  steam 
and  oil  entering  the  body  of  the  burner  separately  and  later 
coming  into  contact  in  such  a  way  that  the  oil  is  literally 
torn  apart  by  the  steam.  This  form  of  burner  has  been 
more  extensively  used  in  the  United  States  than  has  the 
former. 

Oil  burning  shares  with  the  burning  of  powdered  coal, 
the  property  of  permitting  very  accurate  regulation  of  the 
air  supply  to  suit  the  quantity  of  fuel  being  burned.  The 
excess  coefficient  may  therefore  be  maintained  at  a  low  value 
and  the  initial  temperature  may  be  made  correspondingly 
high.  Part  of  the  advantage  thus  gained  over  the  com- 
moner methods  of  coal  firing  is,  however,  counterbalanced 
by  the  quantity  of  steam  used  for  heating  and  pumping  the 
oil  and  for  atomizing  in  some  forms  of  burners. 

Both  oil  burning  and  powdered-coal  burning  can  be 
easily  made  to  give  smokeless  combustion  in  properly 
designed  furnaces  and  both  yield  readily  to  forcing.  That  is, 
the  temporary  consumption  of  excessive  quantities  of  fuel 
to  tide  over  short  demands  for  excessive  amounts  of  steam 
is  comparatively  easily  effected. 

145.  Rate  of  Combustion.  The  rate  at  which  coal  is 
burned  in  a  given  furnace  or  on  a  certain  grate  is  generally 
given  in  terms  of  pounds  cf  coal  fired  per  square  foot  of  grate 
surface  per  hour  and  is  referred  to  as  the  rate  of  combustion. 

The  rate  at  which  coal  can  be  consumed  is  largely 
dependent  on  the  intensity  of  draft  available,  that  is,  on 
the  air  pressure  available  for  driving  air  through  and  over 
the  bed  of  fuel.  The  higher  the  pressure  available,  the 
greater  will  be  the  quantity  of  air  which  can  be  supplied 
and  the  greater  will  be  the  quantity  of  coal  that  can  be 
burned.  If  it  were  not  for  the  cost  of  creating  the  draft, 
the  only  limit  to  increasing  the  rate  of  combustion  would 
occur  when  the  velocity  of  the  air  became  so  great  that  the 


336  STEAM  POWER 

fuel  would  be  picked  up  from  the  grate  and  carried 
onward  into  the  flues  in  a  partly  burned  condition.  Com- 
mercial drafts  give  pressure  differences  above  and  below 
the  fuel  bed  which  range  from  about  0.1  inch  of  water  to 
as  high  as  8  ins.  In  stationary  plants  the  pressures  generally 
range  from  0.1  to  about  0.5  in  cases  where  hand  firing  is 
employed,  and  are  carried  as  high  as  5  or  more  inches 
of  water  with  some  forms  of  mechanical  stokers. 

The  best  rate  of  combustion  varies  with  the  type  and 
size  of  fuel,  the  type  and  size  of  furnace,  the  type  and  size 
of  boiler,  the  draft  and  many  other  considerations.  In  ordi- 
nary power-plant  practice  the  rates  of  combustion  com- 
mercially used  generally  fall  within  the  following  limits: 
with  anthracite,  15  to  20  Ibs.per  square  foot  per  hour;  with 
semi-bituminous,  18  to  22  Ibs.;  and  with  bituminous,  24  to 
32  Ibs.  There  is  a  rapidly  growing  tendency  to  exceed  these 
values,  particularly  in  the  case  of  large  plants. 

As  practically  all  of  the  volatile  is  consumed  above  the. 
grate,  the  fixed  carbon  content  is  practically  the  determining 
factor,  since  it  is  this  constituent  that  is  burned  on  the 
grate.  This  explains  the  high  rate  possible  with  fuels  with 
high  volatile  content.  The  most  economical  results  are 
generally  obtained  when  from  12  to  16  Ibs.  of  fixed  carbon 
are  consumed  per  square  foot  of  grate  per  hour. 

The  figures  given  above  do  not  represent  limiting  con- 
ditions. In  torpedo-boat  practice,  where  high-draft  pres- 
sures are  used  (from  4  to  8  ins.  of  water),  rates  of  from  50 
to  120  Ibs.  are  attained.  On  locomotives,  which  also  use 
high-draft  pressures,  rates  of  combustion  greatly  in  excess 
of  stationary  practice  are  generally  used. 

The  capacity  of  a  given  boiler,  that  is,  its  ability  to 
generate  steam,  increases  as  the  rate  of  combustion  is 
increased,  since  more  heat  is  thus  made  available.  The 
economy  of  the  combination,  that  is,  pounds  of  steam 
generated  per  pound  of  coal  fired,  increases  until  some  best 
rate  of  combustion  for  the  fuel  in  question  is  reached,  and 


STEAM  BOILERS 


337 


thereafter  decreases.  The  variation  of  economy  is,  however, 
not  very  great  for  a  comparatively  wide  range  of  combustion 
on  either  side  of  the  best  rate. 

Curves  giving  approximate  draft  pressures  required  fc.v 
different  rates  of  combustion  when  different  kinds  and  sizes 
of  fuel  are  hand  fired  are  given  in  Fig.  201.  The  sizes 
referred  to  are  explained  in  Tables  XIII  and  XIV.  Table 
XIII  also  shows  the  relative  increase  of  ash  content  as  the 


0  5  10  15  20  25  30  35  40 

Pounds  of  Coal  per  Sq.  Ft.  of  Grate  Surface  per  Hour 

FIG.  201.— Draft  Required  for  Different  Rates  of  Combustion  with 
Different  Sizes  and  Kinds  of  Fuel. 


size  decreases,  there  being  a  tendency  toward  the  concen- 
tration of  the  ash  in  the  smaller  sizes. 

146.  Strength  and  Safety  of  Boiler.  Attention  has 
already  been  called  to  the  fact  that  the  boiling  vessel  has  to 
be  designed  with  two  different  requirements  in  view:  it 
must  be  mechanically  strong  to  resist  internal  pressure  and 
it  must  transmit  the  maximum  amount  of  heat  to  the  con- 
tained water. 

Spherical  and  cylindrical  surfaces  with  the  pressure  act- 


338 


STEAM  POWER 


TABLE  XIII 

SIZES  OF  ANTHRACITE  COAL 

(Sizes  larger  than  pea  coal  generally  too  costly  for  power-plant  use.) 


Through  Screen 

Over  Screen 

Ash  Content 

Name. 

with  Mesh. 

with  Mesh. 

(Average)  . 

(Inclusive.) 

(Inclusive.) 

Run  of  mine           

unscreened 

unscreened 

Broken.          

2f 

Eetr 

2| 

2 

6 

Stove  

2 

11 

10 

Chestnut  

li 

i 

13 

Pea 

3 

\ 

15 

Buckwheat  No.  1  

* 

1 

17 

Buckwheat  No.  2  or  rice.  .  . 

1 
4 

\ 

18 

TABLE  XIV 

SIZES  OF  BITUMINOUS  COALS 

(Considerable  variation  in  commercial  practice  in  naming  and  sizing.) 


Through  Bars 

Over  Bars  Spaced 

Name. 

Spaced  Apart. 

Apart. 

(Inches.) 

(Inches.) 

Lump 

U 

Nut                                   

H 

f 

Slack  

I 

ing  on  the  inside  of  the  curve  are  best  adapted  to  resist 
such  pressures,  as  they  already  have  the  shape  which  the 
pressure  would  tend  to  give  them.  Boilers  are,  therefore, 
constructed  as  far  as  possible  of  vessels  having  only  spherical 
and  cylindrical  surfaces. 

Flat  surfaces  which  are  poorly  adapted  to  resist  such 
pressures  as  act  within  a  boiler  must  often  be  used  despite 
their  weakness.  When  incorporated  in  a  boiler  they  are 
invariably  "  stayed,"  that  is,  braced  by  being  fastened  to 
other  surfaces  by  stay  bolts  and  other  forms  of  fastenings. 
Examples  will  be  given  later. 

Most  of  the  early  designs  of  boilers  and  many  of  the 


STEAM  BOILEES 


339 


modern  types  consist  of  large  cylindrical  vessels  made  by 
riveting  together  properly  shaped  steel  plates.  These  shells 
are  often  traversed  from  end  to  end  by  flues  or  tubes  for 
carrying  hot  gases  and  generally  have  flat  ends  more  or 
less  perfectly  braced  by  these  tubes  and  by  long  tie  rods 


FIG.  202. — Lap  Joint. 


FIG.  203.— Butt  Strap  Joint. 


and  other  braces.     Such  boilers  when  in  operation  are  almost 
entirely  filled  with  water  and  often  hold  many  tons. 

Boilers  of  these  types  have  been  responsible  for  many 
disastrous  boiler  explosions,  and  this  fact  has  led  inventors 
to  the  development  of  models  which  should  be  less  dangerous. 
It  seems  practically  impossible  to  develop  a  commercial 
boiler  which  cannot  be  made  to  explode  to  a  certain  extent 


gritndinal 


FIG.  204.— Riveted  Plates  of  Boiler  Shell. 


FIG.  205. 


if  sufficiently  mistreated  and  mishandled;  but  much  can  be 
done  to  minimize  the  danger. 

The  great  weakness  of  the  older  forms  lies  in  the  riveted 
joints,  which  can  never  be  made  as  strong  as  the  plates 
which  they  fasten  together.  Two  types  of  joint  are  in  use; 
they  are  known  respectively  as  the  lap  joint  and  the  butt 
strap  joint.  These  are  shown  in  Figs.  202,  203  and  204. 


340  STEAM  POWER 

So  far  as  a  circumferential  seam,  that  is,  one  running  around 
the  cylinder  as  shown  in  Fig.  204,  is  concerned,  the  lap  joint 
is  perfectly  satisfactory  and  is  universally  used.  With 
longitudinal  seams,  however,  this  is  not  the  case.  A  lap 
joint  throws  the  joined  edges  out  of  a  true  cylindrical 
surface  as  shown  in  Fig.  205,  and  when  the  vessel  is  subjected 
to  pressure  there  will  be  a  tendency  for  the  plates  to  assume 
a  cylindrical  contour  as  nearly  as  possible.  This  causes 
local  bending  of  the  plates  on  each  side  of  the  lines  of  rivets, 
and  the  continued  repetition  of  this  action  ultimately  causes 
failure.  The  conditions  are  often  made  still  worse  by  calking 
the  joint  on  a  line  indicated  by  a  in  Fig.  205,  that  is,  by 
hammering  the  metal  at  the  inner  surface  of  the  edge  of 
the  outer  plate  into  firmer  contact  with  the  outer  surface 
of  the  inner  plate  for  the  purpose  of  making  a  tight  joint. 

The  butt-strap  joint  can  obviously  be  made  so  that  the 
joined  plates  more  nearly  form  a  true  cylindrical  surface. 

Other  weaknesses  of  the  older  forms  lie  in  the  flat  surfaces 
used;  in  constructions  which  render  it  possible  for  sediment 
to  collect  on  heated  surfaces  and  thus  permit  local  over- 
heating of  the  plate;  and,  above  all,  in  the  very  large 
quantity  of  water  contained. 

The  disastrous  consequences  of  boiler  explosions  are 
generally  due  to  the  action  of  the  hot  water  contained  within 
the  boiler  and  not  to  the  steam  contained  at  the  time  rupture 
occurs.  The  water  within  the  boiler  is  under  steam  pressure 
and  approximately  at  steam  temperature.  Removal  of  the 
pressure  by  rupture  of  the  container  would  enable  a  great 
part  of  this  water  to  flash  suddenly  into  steam  at  the 
expense  of  its  own  heat,  and  this  is  exactly  what  occurs  in 
the  case  of  a  boiler  explosion.  Local  failure  causes  a  sudden 
lowering  of  pressure,  and  this  results  in  the  formation  of 
large  volumes  of  steam  which,  blowing  out  through  the 
initial  fracture,  tend  to  enlarge  it,  to  move  the  boiler  and 
surroundings,  and,  in  general,  to  do  all  possible  to  further 
the  rupture  and  make  conditions  worse. 


STEAM  BOILERS  341 

From   the    preceding    discussion    the   requirements   for 
maximum  safety  can  be  deduced.     They  are: 

1.  The  smallest  convenient  diameter  of  cylindrical  ves- 
sels, so  as  to   decrease    the  total  load  on  joints  for  any 
given  steam  pressure. 

2.  The  elimination  of  the  greatest  possible  number  of 
riveted  joints  and  the  use  of  butt-strap  longitudinal  joints 
on  all  large-diameter,  cylindrical  vessels. 

3.  The  substitution  of  curved  surfaces  for  all  flat  stayed 
surfaces. 

4.  So  shaping  the  boiler  that   the   required   extent   of 
heating  surface  may  be  obtained  without  enclosing  a  great 
volume   to   be   filled   with   hot   water   when   the  boiler  is 
steaming. 

5.  So  shaping  the  boiler  that  such  water  as  is  contained 
therein   will   be    divided   up   into   small   masses   contained 
within  separate  vessels  connected  in  such  a  way  that  rapid 
flow  of  all  water  toward  one  point  of  failure  is  impossible. 

6.  So  shaping  the  boiler  that  no  riveted  joints  shall  be 
in  the  paths  of  flames  and  that  no  sediment  can  collect  on 
metal  immediately  over  flames  or  exposed  to  very  hot  gases. 

7.  So  shaping  the  boiler  that  it  shall 
be    free    to    expand    and    contract    with 
changes   of   temperature,    with  the   least 
resultant  strain  on  the  different  parts. 

These  various  requirements  are  most 
nearly  met  in  the  different  forms  of  water- 
tube  boilers,  some  of  which  will  be  de- 
scribed in  succeeding  paragraphs. 

147.  Circulation  in  Boilers.     If  a  flask 

of  water,  such  as  that  shown  in  Fig.  206,    *?G:  206'~ . 

.     . .  lation  in  a  Flask, 

be  heated  in  the  manner  indicated,   the 

water  will  gradually  acquire  motion  and  follow  paths 
such  as  those  shown  by  the  arrows  in  the  illustration.  The 
heated  water  will  rise  in  the  center  of  the  mass  and  the 
cooler  water  will  flow  downward  around  the  outer  surface. 


342  STEAM  POWER 

Such  motion  is  called  circulation.  Rapid  circulation  within 
a  boiler  is  very  desirable,  since  it  brings  the  maximum  quan- 
tity of  water  in  contact  with  the  heating  surfaces  in  a  given 
time  and  hence  tends  to  increase  the  amount  of  heat  taken 
from  those  surfaces.  It  also  tends  to  sweep  along  any 
bubbles  of  steam  or  gas  formed  on  such  surfaces  and  to  carry 
away  any  sediment  which  may  have  collected,  thus  pre- 
venting overheating  of  the  surfaces. 

Circulation  can  be  expedited  by  providing  free  and 
unrestricted  paths  for  the  water  so  as  to  guide  it  in  the 
proper  directions  and  by  applying  the  most  intense  heat 
at  the  proper  point  along  the  path  of  the  water.  The  tem- 
perature of  the  water  which  is  subjected  to  the  most  intense 


SS 

Blow  off  Valve 

FIG.  207. — Elementary  Types  of  Boilers. 

heat  is  naturally  raised  and  the  water  at  that  point  becomes 
less  dense  than  in  other  parts  of  the  boiler.  The  formation 
of  steam  at  such  points  also  materially  lessens  the  density. 
As  a  result  of  this  lowering  of  density  the  heated  water 
rises  and  the  cooler  water  descends  to  take  its  place.  The 
more  rapid  this  exchange  can  be  made,  the  more  steam  can 
be  generated  from  a  given  amount  of  surface  in  a  given 
time  and  hence,  other  things  equal,  the  better  the  boiler. 

The  elements  of  two  common  forms  of  boiler  are  shown 
in  Fig.  207,  the  arrows  indicating  the  direction  of  the  cir- 
culation and  its  effect  upon  the  delivery  of  steam  and 
of  sediment. 

148.  Types  of  Boilers.  In  a  book  of  this  scope  it  would 
be  impossible  to  describe  all  the  types  of  boilers  at  present 


STEAM  BOILEES 


343 


in  use.     The  more  important  varieties  have  therefore  been 
chosen  for  description  and  illustration. 


Two  types  of  internally  fired,  tubular  boilers  more 
accurately  described  as  internally  fired,  upright  or  vertical, 
fire-tube  boilers  are  shown  in  Fig.  208.  The  furnace  is 


344 


STEAM  POWER 


Tube  Sheet 

Steam  Space 
Exposed  Tubes 

Water  Level 


Water  Column 
and  Try  Cocks 


Feed  Water 
Connection 


Pressure 
Gauge 


Hand  Holes 
(Closed  by 
hand  hole  covers 
i  iu  operation) 


FIG.  209.— Large  Internally  Fired  Tubular  Boiler. 


STEAM  BOILERS  345 

contained  within  the  shell  of  the  boiler  and  is  almost  com- 
pletely surrounded  with  water.  The  heat  radiated  from 
the  hot  fuel  is  thus  almost  entirely  received  by  the  water 
of  the  boiler.  The  hot  gases,  rising  from  the  fuel  bed, 
pass  upward  through  the  tubes  and,  after  giving  up  part 
of  their  heat  to  the  surrounding  metal,  enter  the  smoke 
box  and  pass  directly  to  the  stack. 

Boilers  of  the  type  shown  in  Fig.  208,  (a)  and  (6)  are 
called  exposed-tube  boilers,  because  the  water  level  is  carried 
below  the  tops  of  the  tubes.  The  tubes,  therefore,  extend 
through  the  steam  space  and  act  as  imperfect  superheaters. 
Boilers  of  the  type  shown  in  Fig.  208,  (c) ,  in  which  the  tubes 
do  not  enter  the  steam  space,  but  are  entirely  covered  by 
water,  are  called  submerged-tube  boilers. 

Upright  tubular  boilers  of  the  types  shown  in  Fig.  208 
are  built  by  a  number  of  manufacturers  in  sizes  ranging 
from  about  4  boiler  horse-power  to  about  50  boiler  horse- 
power. They  are  self  contained,  require  no  setting  of  any 
kind,  and  are  shipped  completely  erected.  Such  boilers 
are  very  often  mounted  on  trucks  or  skids  and  used  to 
generate  steam  for  small  hoisting  and  other  forms  of  con- 
tractors' engines.  They  are  also  used  on  steam  fire  engines. 

The  pressure  carried  in  these  small  tubular  boilers  is 
generally  under  100  Ibs.  per  square  inch,  but  they  can  be 
built  for  higher  pressures  if  desired. 

In  Fig.  209  is  shown  a  larger  type  of  internally  fired 
tubular  boiler  as  made  by  the  Bigelow  Company  for  station- 
ary use.  These  boilers  are  similar  to  those  just  described, 
but  are  made  only  in  large  sizes,  in  this  case,  in  sizes  ranging 
from  40  boiler  horse-power  to  200  boiler  horse-power.  The 
exposed  tubes  generally  give  a  superheat  of  about  25°  F. 

These  large  upright  boilers  can  be  built  to  operate  with 
a  pressure  as  high  as  200  Ibs.  per  square  inch  and  because 
of  the  small  area  covered  by  even  the  largest  sizes,  they 
are  particularly  adapted  to  locations  in  which  floor  space 
is  limited. 


346 


STEAM  POWER 


The  locomotive  type  of  boiler  is  shown  in  Fig.  210.  It  is 
an  internally  fired,  horizontal,  tubular  or  fire-tube,  boiler. 

Such  boilers  are  seldom  used  for  stationary  purposes, 
but  are  universally  used  on  steam  locomotives  and,  in  the 
smaller  sizes,  are  often  mounted  on  trucks  or  skids  arid  used 
for  semi-stationary  purposes  by  contractors  and  others. 
Boilers  of  this  type  are  built  in  sizes  ranging  from  10  boiler 
horse-power  or  less  up  to  over  100  boiler  horse-power  for 
general  power  purposes,  while  those  used  on  the  largest 
locomotives  generate  over  2000  boiler  horse-power. 

The  Continental  type  of  boiler,  named  from  the  Con- 
tinental Iron  Works,  is  shown  in  Fig.  211.  These  boilers 


Handholes 

FIG.  210. — Locomotive  Type  of  Boiler. 

may  be  described  as  internally  fired,  return  tubular,  with 
semi-external  combustion  chamber,  this  chamber  being  out- 
side of  the  boiler  shell  proper  but  being  built  as  an  integral 
part  of  the  boiler  and  transportable  therewith.  Boilers  of 
this  type  are  built  in  sizes  ranging  from  about  75  boiler 
horse-power  to  300  or  more. 

The  grates,  furnace  and  ash  spaces,  and  bridge  wall  are 
all  carried  within  circular,  corrugated  flues,  one  flue  being 
used  in  the  smaller  sizes  and  two  in  the  larger.  The  corru- 
gations serve  the  double  purpose  of  strengthening  the  flue 
and  of  exposing  added  heating  surface  to  fire  and  hot  gases. 

The  steam  pipe  shown  just  below  the  steam  connection 
at  the  top  of  the  boiler  is  commonly  used  on  boilers  for  the 


STEAM  BOILERS 

noipsg  dox  i 


347 


348 


STEAM  POWER 


purpose  of  preventing  the  escape  of  excessive  quantities  of 
moisture  with  the  steam. 

These  boilers  are  very  compact  in  shape  and  are  short 
for  their  capacity,  but  they  contain  a  great  volume  of  water. 
They  possess  the  advantages  of  having  a  large  steam  space 
and  a  very  extended  liberating  surface  over  which  the  steam 
separates  from  the  water. 


Uptake 


Tubes 


Fire  Doors 


Ash  Pits 


Man  Hole- 

FIG.  212.— Scotch  Marine  Type  Boiler. 

The  Scotch  marine  type  of  boiler  is  shown  in  Fig.  212. 
It  has  the  same  general  construction  as  that  just  described 
excepting  that  the  combustion  chamber  is  entirely  enclosed 
within  the  water  space  of  the  boiler.  This  chamber  is 
built  up  of  flat  plates  and  is  held  against  collapse  by  numer- 
ous stay  bolts.  Boilers  of  this  type  were  until  recently 
the  standard  for  marine  practice,  but  they  are  now  being 
replaced  in  many  instances  by  water-tube  boilers  of  more 
recent  design. 


STEAM  BOILERS 


349 


Scotcn  marine  boilers  are  very  economical  in  the  use  of 
fuel,  are  good  steamers,  and  are  absolutely  self  contained. 
They  are  built  in  numerous  sizes,  the  smallest  having  shells 
with  diameters  of  about  6  ft.,  while  the  largest  diameter 
used  is  about  16  ft.  The  largest  boilers  have  three  and 
four  corrugated  furnaces. 

Two  types  of  externally  fired,  return-tubular  (or 
"  H.R.T.")  boilers  are  shown  in  Figs.  213  and  214.  The 


Manhole 


Stay  RocUr  ~ 


Front  ^lU=^r  3      - — "^ShBuck  Stave 

FIG.  213.— Horizontal  Return-tubular  Boiler  with  "  Full  Flush  Front." 

only  essential  differences  in  these  two  types  are  in  the  forms 
of  setting  and  in  the  methods  of  suspending  the  boilers. 
The  shell  is  generally  rigidly  supported  at  the  furnace  end 
and  arrangements  made  to  allow  for  movement  of  the  other 
end  with  changes  of  temperature. 

These  boilers  can  be  built  very  cheaply  and  are  therefore 
widely  used  when  their  limitations  do  not  prevent.  It  has 
been  found  inadvisable  to  build  them  in  sizes  larger  than 
200  boiler  horse-power  or  for  pressures  higher  than  150 
Ibs.  per  square  inch,  and  they  are  generally  used  in  smaller 


350 


STEAM  POWER 


STEAM  BOILEES 


351 


352 


STEAM  POWEK 


FIG.  216.— Forged 
Header  for  Bab- 
cock  &  Wilcox 
Boiler. 


sizes  and  with  lower  pressures.  These  limitations  are  set 
by  permissible  thickness  of  metal  immediately  above  the 
fire,  experience  having  shown  that  the 
plates  deteriorate  rapidly  at  this  point  if 
made  too  thick. 

One  form  of  Babcock  &  Wilcox  water- 
tube  boiler  is  shown  in  Fig.  215.  This 
boiler  is  built  up  of  sections  consisting  of 
several  tubes  joined  at  the  ends  by  headers, 
and  the  sections  are  connected  side  by  side 
at  each  end  to  a  long  horizontal  drum. 
The  ends  of  this  drum  are  closed  with 
"  dished  "  heads,  thus  doing  away  with 
flat  surfaces  and  the  necessity  for  stays 
within  the  drum. 

A  detail  of  the  forged  header  is  shown  in 
Fig.  216.  It  may  be  regarded  as  a  long 
box  of  rectangular  section  with  opposite 
walls  pierced  by  circular  holes,  which  has  been  so  distorted 
as  to  give  it  a  wavy  shape.  The  distortion  brings  the  holes 
into  such  positions  that  the  tubes  when  expanded  into  these 
holes  are  "  staggered,"  that  is, 
do  not  lie  one  above  the  other. 
The  general  principle  in- 
volved in  the  arrangement  of 
these  sections  or  elements  and 
the  resulting  circulation  are 
shown  in  Fig.  217.  The  location 
of  the  feed-water  inlet  and  other 
details  are  shown  in  Fig.  218. 
It  will  be  observed  that  the  feed  FIG.  217.— Elementary  Babcock 
water  enters  in  such  a  direction  *  Wilcox  Boiler,  Showing 

,,,.,.  ,.,  Circulation, 

and  position  that  it  is  readily 

picked  up  by  the  current  of  water  circulating  in  the  boiler, 
carried  toward  the  rear  and  down  the  rear  header.  During 
this  travel  it  is  heated  by  contact  with  the  hot  water  in 


STEAM  BOILERS 


353 


Hand  hole  opposite  encL 
of  tube,  closed  by  hand 
hole  cover  when  in 
operation. 


End  of  tube 
expanded  into 
hole  of  header. 


FIG.  218. — Details  of  Babcock  &  Wilcox  Boiler  Construction. 


354 


STEAM  POWER 


the  boiler  and  most  of  its  impurities  are  separated  out  and 
settle  in  the  mud  drum  at  the  bottom  of  the  rear  header. 


The  boiler  is  suspended  by  stirrups  from  beams  carried 
by  the  brickwork  as  shown  in  Fig.  215,  the  tube  sections 


STEAM  BOILERS 


355 


simply  hanging  from  the  drum  by  the  nipples  at  each  end. 
The  various  parts  of  the  structure  are  thus  free  to  expand 
and  contract  independently  as  their  temperatures  change 
and  are  not  bound  in  any  way  by  the  brick  setting. 

The  steam  is  collected  from  a  perforated  steam  pipe  near 
the  top  of  the  steam  space.  The  baffle  shown  in  Fig.  218 
prevents  the  steam  which  rises  from  the  front  header  from 
carrying  the  water  bodily  into  the  steam  space  and  makes 
the  greater  part  of  the  water  surface  in  the  drum  act  as 
separating  surface. 

The  scale  which  accumulates  inside  of  the  tube  is  removed 
by  tools  inserted  through  the  hand  holes  in  the  front  headers 
opposite  the  ends  of  the  tubes.  One  of  these  hand  holes 
and  its  cover  are  shown  in  section  in  Fig.  218.  Soot  .and 
dust  which  accumulate  on  the  outer  surfaces  of  the  tubes 
are  blown  off  periodically  by  a  steam  jet,  the  necessary 
nozzle  and  hose  being  "in- 
serted through  the  tall  and 
narrow  side  cleaning  doors 
shown  in  Fig.  215  opposite 
each  "  pass." 

A  section  of  the  Heine 
water-tube  boiler  is  shown 
in  Fig.  219.  This  boiler  con- 
sists of  a  slightly  inclined 
drum  with  dished  heads,  two 
sheet-steel  headers  and  nu- 
merous tubes  connecting 
these  headers.  The  shape 
of  the  header  is  shown  in 
Fig.  220,  which  indicates  the 
positions  occupied  by  the  tubes  and  the  way  in  which 
the  header  is  joined  to  the  drum. 

The  products  of  combustion  are  generally  made  to  pass 
along  the  tubes  by  the  longitudinal  baffles  shown,  instead 
of  across  the  tubes  as  in  the  boiler  last  described. 


<£>OOOOOOOOOOO 


FIG.  220. — Front  End  Elevation, 
Heine  Boiler. 


356  STEAM  POWER 

The  mud  drum  in  this  type  is  located  within  the  boiler 
and  consists  of  a  sheet-steel  box  supported  a  few  inches 
above  the  bottom  of  the  drum.  The  feed  water  enters  at 
the  front  end  of  this  drum  and  gradually  spreads  out  as  it 
is  heated  by  the  surrounding  water.  The  greater  part  of 
the  impurities  settles  to  the  bottom  and  is  blown  off  period- 
ically. The  warmed  water  rises  and  flows  out  of  an  opening 
in  the  top  of  the  box  at  the  front  end  and  there  joins  the 
circulation  of  the  boiler,  traveling  toward  the  rear,  down 
the  rear  header  and  up  the  tubes  to  the  front  header. 

The  interior  of  the  tubes  is  cleaned  of  scale  through 
hand  holes  just  as  in  the  last  boiler.  The  external  surfaces 
are  freed  of  soot  and  dust  by  means  of  a  steam  jet  which 
is  introduced  through  the  stay  bolts  in  the  headers,  these 
bolts  being  made  hollow  for  this  purpose.  Since  it  is  not 
necessary  to  use  doors  in  the  side  walls  for  cleaning  in  this 
type,  Heine  boilers  are  often  set  up  in  batteries  of  three  or 
more,  each  interior  side  wall  serving  as  the  side  wall  of  two 
settings.  In  the  case  of  the  boiler  last  described  the  neces- 
sity for  side  cleaning  doors  makes  it  impossible  to  join  more 
than  two  boilers  in  this  way. 

The  Heine  boiler  is  supported  by  standing  the  front 
and  rear  headers  upon  the  brickwork  of  the  setting  and 
it  can  therefore  expand  freely  in  all  directions. 

A  section  of  the  Sterling  water-tube  boiler  is  shown  in 
Fig.  221.  This  boiler  consists  of  three  upper  horizontal 
drums  connected  by  short  curved  tubes  and  connected  to 
a  single  lower  horizontal  drum  by  means  of  long  tubes  which 
are  curved  near  the  ends.  The  curves  of  all  tubes  are  so 
made  that  the  tubes  enter  the  drum  surfaces  radially,  thus 
giving  a  simple  joint  which  is  readily  made  tight  by  expand- 
ing the  tube  into  the  sheet. 

The  feed  water  is  introduced  into  the  upper  rear  drum, 
and  is  gradually  heated  and  partly  purified  as  it  passes 
downward  to  the  lower  drum,  in  which  the  greater  part  of 
the  material  precipitated  from  the  water  is  caught  and 


STEAM  BOILEES 


357 


stored  until  blown  off.  From  the  lower  drum  the  water 
is  supposed  to  pass  upward  through  the  front  bank  of  tubes, 
the  steam  formed  passing  to  the  central  drum  through  the 
upper  set  of  short  curved  tubes,  and  the  water  which  is 
not  evaporated  passing  to  the  central  drum  through  the 


Feed  Wator  Inlet 


Steam  Connection. 


Smolc 
Couucc (ion 


Damper 


Bottom  Blow-off 

FIG.  221 


-Section  of  Sterling  Boiler. 


lower  set  of  curved  tubes.  This  water  passes  from  the  upper 
central  drum  to  the  lower  and  returns  through  the  front 
bank  of  tubes.  Any  steam  formed  in  the  rear  bank  of  tubes 
or  in  the  rear  drum  passes  to  the  central  drum  through  the 
short  curved  tubes  connecting  the  steam  spaces. 

The  entire  boiling  vessel  is  hung  from  a  frame  of  struc- 
tural steel  by  means  of  the  upper  drums,  so  that  the  lower 


358  STEAM  POWER 

drum  hangs  practically  free  on  the  tubes.  Independent 
expansion  of  all  the  members  is  insured  by  this  method 
of  suspension  and  by  the  curvature  of  the  tubes,  which  per- 
mits each  one  of  them  to  bend  to  the  extent  necessary  to 
equalize  any  strains  caused  by  changing  temperatures. 

The  interiors  of  the  tubes  are  cleaned  by  means  of  tools 
lowered  from  inside  the  upper  drums  and  the  exterior 
surfaces  are  blown  off  by  steam  jets  introduced  through 
doors  in  the  brickwork  of  the  setting. 

The  Wickes  vertical  water-tube  boiler  is  shown  in 
section  in  Fig.  222.  It  consists  of  an  upper  and  lower  cir- 
cular drum,  connected  by  straight  tubes  expanded  into  the 
lower  and  upper  heads  of  the  drums  respectively.  A 
vertical  baffle  placed  in  the  center  of  the  bank  of  tubes 
gives  an  upward  path  to  the  products  of  combustion  when 
passing  over  the  front  tubes  and  a  downward  path  when 
passing  over  the  rear  tubes. 

The  feed  water  is  generally  introduced  at  the  rear  of 
the  upper  drum,  the  circulation  being  downward  in  the  rear 
tubes  and  upward  in  the  front  tubes. 

The  interior  surfaces  of  the  tubes  are  cleaned  by  tools 
lowered  into  them  by  a  man  standing  within  the  upper 
drum,  which  is  made  high  enough  to  make  this  possible. 
The  external  surfaces  are  cleaned  by  steam  jets  inserted 
through  doors  in  the  brickwork. 

The  entire  boiler  is  supported  on  brackets  riveted  to 
the  lower  or  mud  drum  and  is  free  to  expand  in  all  directions, 
the  brickwork  simply  enclosing  but  not  confining  it. 

149.  Boiler  Rating.  Practically  all  apparatus  which  is 
connected  with  the  development  of  power  is  given  a  horse- 
power rating.  In  some  cases  such  a  method  of  rating  is 
convenient  and  simple,  in  others  it  is  inconvenient,  irra- 
tional and  complicated.  The  term  horse-power,  when  used 
as  a  measure  of  work  or  power,  means  very  definitely  the 
equivalent  of  33,000  ft.-lbs.  per  minute.  When,  however, 
a  certain  number  of  horse-power  is  used  as  the  rating  of  a 


STEAM  BOILERS 


359 


team  Connection 


*^Feed  Water 
«*  Connection 


ttom 
Blow-off 


FIG.  222.— Wickes  Vertical  BoUer. 


360  STEAM  POWER 

particular  piece  of  apparatus,  it  generally  means  that  that 
piece  of  apparatus,  when  working  at  about  its  best  effi- 
ciency, can  do  what  is  necessary  to  make  available  the 
stated  number  of  horse-power  in  the  plant  of  which  it  forms 
a  part. 

Thus  a  boiler  rated  at  a  certain  horse-power  was  origi- 
nally supposed  to  be  able  to  supply  the  amount  of  steam 
required  by  an  average  engine  developing  that  quantity 
of  power  and  to  do  this  when  working  at  its  best  efficiency. 
The  water  rates  of  engines  are,  however,  so  different  that 
there  is  no  real  connection  between  boiler  horse-power  and 
engine  horse-power,  and  it  is  best  to  consider  the  boiler 
horse-power  as  a  perfectly  arbitrary  unit  defined  in  a  certain 
way. 

The  American  Society  of  Mechanical  Engineers  has 
defined  the  boiler  horse-power  as  the  equivalent  of  the  evapora- 
tion 0/34.5  /6s.  of  water  per  hour  from  and  at  212°  F.  This 
means  the  conversion  per  hour  of  34.5  Ibs.  of  water  at  212°  F. 
into  steam  at  the  same  temperature  and  therefore  at  atmos- 
pheric pressure. 

Each  pound  of  steam  generated  under  these  conditions 
requires  the  expenditure  of  the  latent  heat  of  vaporization 
rt  atmospheric  pressure,  which  is  equal  to  970.4  B.t.u. 
according  to  the  latest  steam  tables.  The  older  tables  gave 
965.7.  This  quantity  of  heat  is  known  as  a  Unit  of  Evapora- 
tion and  is  abbreviated  U.E.  The  boiler  horse-power  is, 
therefore,  the  equivalent  of  34.5  U.E.  per  hour  or  34.5X970.4 
=  33,479  B.t.u.  per  hour. 

As  practically  no  power-plant  boilers  receive  their  feed 
water  at  a  temperature  of  212°  F.  and  convert  it  into  steam 
at  the  same  temperature,  it  is  necessary  to  convert  the 
weight  actually  evaporated  to  what  it  would  have  been 
from  and  at  212°  F.  and  then  to  divide  this  figure  by  34.5 
in  order  to  find  the  boiler  horse-power  developed. 

The  number  of  pounds  which  would  have  been  evaporated 
from  and  at  212°  F.  if  the  same  amount  of  heat  had  been 


STEAM  BOILERS  361 

transmitted  is  known  as  the  equivalent  evaporation,  or  as 
the  equivalent  weight  of  water  evaporated  into  dry  steam  from 
and  at  212°  F. 

The  method  of  obtaining  the  equivalent  evaporation 
has  been  defined  by  the  American  Society  of  Mechanical 
Engineers.  The  heat  given  to  each  pound  of  dry  saturated 
steam  produced  is  to  be  determined;  this  is  to  be  multiplied 
by  the  total  weight  of  dry  saturated  steam  generated  per 
hour,  and  the  product  is  to  be  divided  by  the  latent  heat 
of  vaporization  at  212°  F.  Thus,  for  a  boiler  receiving 
its  feed  water  at  some  temperature  tf  above  32°  F.,  the 
water  contains  a  quantity  of  heat  equal  to  <//  B.t.u.  per 
pound,  g/  being  found  in  the  steam  table  opposite  the  tem- 
perature tf.  Each  pound  of  dry  saturated  steam  leaving 
the  boiler  carries  with  it  an  amount  of  heat  equal  to  X  for 
the  existing  temperature.  The  heat  supplied  each  pound 
in  the  boiler  must  therefore  be  X—  qf  and,  for  W  pounds 
per  hour,  the  heat  supplied  would  be  W(\—q/).  The 
equivalent  evaporation  is  then  given  by 

Equiv.  evap.  =  TF(=~n  Ibs.  per  hour.     .     (101) 


This  expression  may  be  regarded  as  consisting  of  two 
factors,  the  weight  of  dry  steam  generated  per  hour,  and 
a  fraction  which  will  always  have  the  same  value  for  a  given 
combination  of  pressure  and  feed-water  temperature.  This 
fraction  is  called  the  factor  of  evaporation,  and  it  is  cus- 
tomary to  tabulate  the  various  values  of  the  factor  of 
evaporation  for  different  common  combinations  of  pressure 
and  feed-water  temperature. 

It  should  be  noted  that  the  equivalent  evaporation  as 
defined  above  gives  the  boiler  no  credit  for  heat  given  to 
water  which  leaves  the  boiler  as  water,  nor  does  it  give 
credit  for  any  superheating.  The  former  may  be  justified 
by  saying  that  the  boiler,  as  a  commercial  piece  of  apparatus, 
is  not  intended  to  supply  hot  water;  but  many  commercial 


362  STEAM  POWEE 

boilers  are  expected  to  supply  superheated  steam  and  should 
be  given  credit  for  heat  used  in  that  way. 

Returning  now  to  the  boiler  horse-power,  its  value  can 
obviously  be  found  for  any  given  boiler  by  dividing  the 
equivalent  evaporation  per  hour  by  the  number  34.5. 

Boilers  are  supposed  to  be  so  rated  that  they  will 
develop  their  rated  horse-power  when  operating  at  about 
their  best  efficiency  and  will  do  it  with  moderate  draft 
and  reasonably  good  firing  with  average  fuel.  Experience 
has  shown  that  for  most  boilers  the  best  efficiency  is  ob- 
tained when  an  equivalent  evaporation  of  from  3  to  3.5  Ibs. 
of  water  occurs  per  square  foot  of  heating  surface.  The 
heating  surface  is  generally  taken  as  the  total  surface  in 
contact  with  hot  gases  excepting  in  the  case  of  tubes.  The 
outer  surfaces  of  tubes  are  generally  counted  even  if  they 
be  in  contact  with  the  water.  An  equivalent  evaporation 
of  3  to  3.5  Ibs.  per  square  foot  would  call  for  a  heating 
surface  of  from  12  to  10  sq.ft.  per  boiler  horse-power. 

Most  water-tube  boilers  are  given  10  sq.ft.  of  heating 
surface  per  rated  boiler  horse-power,  and  most  return- 
tubular  boilers  are  supplied  with  11  to  12  sq.ft.  Scotch 
marine  boilers  are  generally  designed  on  a  basis  of  about 
8  sq.ft.  per  rated  boiler  horse-power. 

The  quantity  of  water  which  can  be  evaporated  per 
square  foot  seems  to  depend  to  a  great  extent  upon  the  rate 
at  which  hot  gases  can  be  passed  over  the  heating  surface, 
and  experiments  have  shown  that  from  five  to  eight  times 
the  ordinary  rates  of  evaporation  can  be  attained  if  suf- 
ficient fuel  can  be  burned.  As  the  rate  of  evaporation  per 
square  foot  is  increased  above  the  commonly  accepted 
value,  the  efficiency  decreases,  but  the  decrease  is  generally 
small  for  a  considerable  increase  in  rate  of  evaporation. 
Most  power-plant  boilers  can  give  from  150  to  200  per  cent 
of  their  normal  rating,  and  some  are  now  being  installed  to 
operate  for  long  periods  at  about  200  per  cent  of  what 
would  be  considered  a  normal  rating. 


STEAM  BOILERS  363 

150.  Boiler  Efficiencies.  There  are  a  great  many  pos- 
sible efficiencies  which  may  be  considered  in  connection 
with  boiler  tests.  The  two  most  commonly  used  are  defined 
by  the  A.S.M.E.,  and  are: 

1.  Efficiency  of  the  boiler 

Heat  absorbed  per  pound  of  combustible  burned 
Calorific  value  of  1  Ib.  of  combustible 

2.  Efficiency  of  boiler  and  grate 

_  Heat  absorbed  per  pound  fuel 
Calorific  value  of  1  Ib.  of  fuel* 

The  names  used  are  not  very  well  chosen,  and  it  is  better  to 
call  the  first  the  efficiency  based  on  combustible  and  the 
second  the  efficiency  based  on  coal.  The  weight  of  com- 
bustible burned  is  calculated  by  subtracting  from  the  coal 
fired  the  total  weight  of  moisture  and  the  total  weight  of 
refuse  in  the  ash  pit. 

The  heat  absorbed  is  by  definition  the  heat  absorbed 
by  the  dry  steam  made  by  the  boiler,  but  it  seems  probable 
that  this  will  also  be  modified  in  the  near  future  as  suggested 
in  a  preceding  paragraph. 

It  is  also  possible  to  determine  the  efficiency  of  the  grate, 
of  the  furnace,  and  of  the  boiling  vessel,  and  this  is  some- 
times done. 

The  best  commercial  operating  values  for  the  efficiency 
of  the  boiler  as  a  whole,  that  is,  the  boiler  and  grate  on  the 
basis  of  total  fuel  fired,  are  about  75  per  cent  for  good 
qualities  of  coal  and  80  per  cent  for  oil,  but  such  values 
are  generally  obtained  only  in  well-equipped  plants  operat- 
ing on  comparatively  constant  loads.  Average  commercial 
values  generally  range  from  60  to  70  per  cent  on  a  yearly 
basis  in  well-equipped  plants  which  are  carefully  operated, 
and  many  boiler  plants  are  operated  at  an  efficiency  of  50 
per  cent  and  less. 

The   pounds   of  water  evaporated   per  pound  of  coal 


364 


STEAM  POWER 


fired  generally  ranges  between  6  and  10,  and  the  equiva- 
lent evaporation  per  pound  of  combustible  burned  will 
generally  fall  between  8  and  12  pounds. 

151.  Effects  of  Soot  and  Scale.  The  flue  gases  in  real 
boilers  are  seldom  clean  mixtures  of  the  products  of  com- 
bustion and  nitrogen,  as  theory  would  indicate.  They 
always  contain  more  or  less  soot  and  unburned  hydro- 
carbons, as  well  as  some  finely  powdered  ash  and  fuel.  With 
strong  draught,  very  large  particles  of  ash  and  fuel  may  be 
carried  by  the  flue  gases. 

These  materials  are  partly  carried  up  the  stack  by  the 
gases  and  partly  deposited  on  the  heating  surfaces  of  the 
boiler.  Such  deposits  decrease  the  conductivity  of  the 
heating  surfaces,  and  if  the  deposits  are  heavy  the  loss  may 
be  very  great.  The  results  of  one  investigation  on  the 
effect  of  soot  are  given  in  Table  XV,  the  values  being 
taken  from  an  article  published  in  the  Proceedings  of  the 
Institute  of  Marine  Engineers  for  the  year  1908.  These 
values  are  probably  too  high,  particularly  for  the  thicker 
deposits,  but  they  serve  to  bring  out  the  fact  that  a  very 
appreciable  loss  does  occur  from  the  presence  of  such 
deposits. 

TABLE  XV 

EFFECT  OF  SOOT  DEPOSITS  ON  BOILER  HEATING  SURFACES 


Thickness  of  Deposit 
in  Inches. 

Loss  of  Conductivity 
in  Per  cent. 

0 

0.0 

& 

9.5 

^ 

26.2 

i 

45.2 

A 

69.0 

The  effect  of  soot  deposits  in  decreasing  the  efficiency 
of  boilers  was  used  for  a  long  time  as  a  basis  for  argument 
in  favor  of  certain  types  of  boilers  in  which  the  heating 
surfaces  were  so  shaped  and  located  that  such  deposits 


STEAM  BOILERS  365 

formed  to  a  minimum  degree  and  against  other  types 
less  favorably  designed  from  this  point  of  view.  Prac- 
tically, however,  the  removal  of  such  deposits  by  means  of 
steam  jets  applied  at  regular  intervals  is  so  simple  that  this 
consideration  need  be  given  little  weight  in  the  selection 
of  a  boiler.  Provision  should  always  be  made,  however, 
for  the  easy  use  of  the  jets  for  cleaning  purposes. 

152.  Scale.  Practically  all  water  available  for  boiler  feed 
contains  various  salts  in  solution  and  it  often  contains  solid 
matter  in  suspension  as  well.  This  material  is  all  deposited 
within  the  boiler  as  the  water  is  heated  and  converted  into 
steam.  There  is  thus  a  gradual  collection  within  the  boiler 
of  all  the  solid  material  brought  in  by  the  water. 

In  well-designed  boilers  the  greater  part  of  such  deposits 
is  carried  to  a  part  of  the  boiler  in  which  the  metallic  sur- 
faces are  not  exposed  to  high  temperature  gases,  as,  for 
instance,  the  mud  drums  in  water-tube  boilers.  It  can  then 
be  drawn  off  periodically  in  the  form  of  a  thin  mud  sus- 
pended in  water.  In  practically  all  boilers,  however,  some 
of  the  solid  material  will  be  carried  to  the  heating  surfaces 
exposed  to  high  temperature  gases  and  deposited  there. 
Under  the  action  of  heat,  the  mud-like  material  gradually 
changes  until,  in  many  instances,  it  forms  a  very  hard, 
stone-like  coating  on  the  heating  surface.  This  is  known 
as  boiler  scale. 

Such  deposits  may  cause  two  kinds  of  trouble:  They 
may  decrease  the  conductivity  of  the  heating  surfaces  and 
thus  decrease  the  efficiency  of  the  boiler;  and,  because  of 
their  location  on  the  water  side  of  the  metal,  they  may  per- 
mit the  hot  gases  to  overheat  that  metal,  thus  weakening 
it.  Such  overheated  metal  often  "  bags  "  under  the  high 
internal  pressure  and  may  eventually  give  way  with  disas- 
trous results.  The  mechanical  structure  of  the  scale  seems 
to  be  the  determining  factor;  scales  which  are  easily  pene- 
trated by  water  have  little  effect,  while  those  which  are  very 
dense  and  non-permeable  may  cause  serious  trouble. 


366  STEAM  POWER 

Boilers  should  be  blown  down  periodically  to  keep  them 
as  free  as  possible  of  scale-forming  material,  and  they  should 
be  so  constructed  that  scale  which  has  been  formed  can  be 
removed  easily.  Very  efficient  tools  have  been  developed 
for  removing  scale  from  the  interior  and  exterior  surfaces 
of  tubes,  so  that  boilers  using  tubular  heating  surfaces  are 
readily  cleaned  of  scale. 

153.  Scale    Prevention.     Much   of    the    solid    material 
carried  by  water  is  deposited  when  the  water  is  heated  to 
a  temperature  of  from  150°  to  200°  F.,  so  that  heating  feed 
water  before  it  is  admitted  to  the  boiler  is  at  least  a  partial 
preventive  in  most  cases. 

Nearly  all  of  the  salts  which  are  soluble  in  hot  water 
and  therefore  are  not  deposited  when  the  feed  is  heated, 
can  be  made  to  form  insoluble  compounds  by  the  addition 
of  comparatively  cheap  chemicals.  By  the  addition  of  such 
chemicals  in  the  feed-water  heaters,  or  in  other  apparatus 
specially  designed  for  that  purpose,  the  greater  part  of  the 
solid  content  of  the  water  can  be  precipitated  before  it  is 
admitted  to  the  boiler. 

There  are  a  great  many  "  boiler  compounds  "  on  the 
market  which  are  intended  to  be  mixed  with  the  water  as 
it  is  fed  to  the  boiler  and  are  supposed  to  prevent  the  for- 
mation of  scale  on  the  heating  surfaces.  All  they  can 
possibly  do  is  to  change  the  chemical  composition  of  the 
solids;  they  cannot  prevent  the  deposit  of  these  solids  within 
the  boiler.  They  are  therefore,  at  best,  only  an  imperfect 
remedy. 

154.  Superheaters.     Many   boiler   plants    are   now  ar- 
ranged to  supply  steam  superheated  25  to  200  degrees  Fahr. 
It  was  shown  in  an  earlier  chapter  that  the  use  of  super- 
heated steam  greatly  improves  the  economy  of  reciprocating 
engines  and  turbines,  and  there  are  also  other  advantages 
which  accrue  from  its  use. 

Superheaters  are  of  two  kinds — separately  fired  and 
built-in  superheaters.  The  separately  fired  superheaters  are 


STEAM  BOILERS  367 

enclosed  in  a  brick  setting  fitted  with  grate  and  furnace 
similar  to  that  of  an  ordinary  boiler.  The  built-in  super- 
heaters are  installed  within  the  boiler  setting  so  that  the 
products  of  combustion  pass  over  them  in  flowing  through 
the  boiler. 

In  either  type  the  steam  passes  through  the  superheater 
on  its  way  from  the  boilers  to  the  engines.  In  the  case 
of  separately  fired  superheaters,  the  temperature  of  the 
superheated  steam  is  controlled  by  regulation  of  the  fire 
on  the  grate  of  the  superheater,  but  in  the  built-in  type 
regulation  in  this  way  is  practically  impossible,  as  the  fire 
under  the  boiler  must  be  controlled  to  suit  the  demand  for 
steam.  The  control  of  such  superheaters  is  therefore  effected 
either  by  locating  them  in  such  a  position  that  the  natural 
variation  in  the  temperature  of  the  gases  reaching  them 
gives  an  approximate  regulation,  or  they  are  installed  in 
a  separate  chamber  and  hot  gases  passed  over  them  in  such 
proportions  as  necessary  to  give  the  required  temperature. 

The  Babcock  &  Wilcox  superheater  as  applied  to  the 
boiler  of  the  same  make  is  shown  in  Fig.  223.  The  steam 
collected  in  the  dry  pipe  within  the  drum  passes  downward 
to  the  upper  manifold  of  the  superheater  and  from  there 
it  flows  through  the  U-shaped  tubes  into  the  lower  manifold. 
From  the  lower  manifold  it  flows  through  the  superheater 
stop  valve  to  the  engine  or  turbine. 

The  superheater  is  so  located  that  the  hot  gases  pass 
over  it  between  the  first  and  second  passes  and  there  is  no 
way  of  shutting  off  these  gases.  Provision,  as  shown  in 
the  illustration,  is  therefore  made  for  flooding  the  super- 
heater during  starting,  or  when  superheated  steam  is  not 
desired.  When  flooded  it  becomes  heating  surface  similar 
to  that  of  the  tubes  below,  the  steam  made  passing  into 
the  drum  through  the  dry  pipe. 

The  Heine  superheater  as  applied  to  a  Heine  boiler  is 
shown  in  Fig.  224.  It  consists  of  a  sheet-metal  header  or 
box  into  which  U-shaped  tubes  are  expanded.  The  steam 


368 


STEAM  POWER 


Safety  Val 


Drain 
Valve 


Front 


Gases 
from  Furnaces 


FIG.  223. 


-Superheated  Steam  from 
Superheater 


Saturated 

Steam  to 

Superheate 

Dam 
Cont 


FIG.  224. — Heine  Superheater. 


STEAM  BOILERS 


369 


enters  the  bottom  of  the  header  and  is  guided  by  dia- 
phragms in  such  a  way  that  it  passes  through  the  lower  set 
of  U-tubes,  returns  to  the  header,  passes  through  the  upper 
set  of  tubes,  and  then  leaves  the  superheater  at  the  top. 


FIG.  225.— H.R.T.  Boiler  and  Foster  Superheater. 

This  apparatus  is  installed  in  a  brick  chamber  built  into 
the  boiler  setting  and  connected  with  the  furnace  by  a  flue 
(not  shown)  in  the  brick  side  wall.  A  damper  controls  the 
flow  of  hot  gases  to  this  chamber  and  the  degree  of  superheat 
is  controlled  by  the  position  of  this  damper. 


FIG.  226. — Element  of  Foster  Superheater. 

The  Foster  superheater  is  shown  installed  in  the  setting 
of  an  H.R.T.  boiler  in  Fig.  225  and  the  details  of  the  con- 
struction of  one  element  are  shown  in  Fig.  226.  The  core 
is  used  to  spread  the  steam  in  a  thin  stream,  thus  bringing 
it  into  better  contact  with  the  heating  surface.  The  fins 


370  STEAM  POWER 

on  the  exterior  of  the  element  are  used  for  the  purpose  of 
getting  a  more  extended  metallic  surface  in  contact  with 
the  hot  gases. 

155.  Draft  Apparatus.  Attention  was  called  in  a  pre- 
ceding paragraph  to  the  fact  that  there  must  be  a  difference 
of  pressure  between  the  spaces  below  and  above  the  fuel 
bed  in  order  to  cause  the  necessary  air  to  flow  through  the 
bed.  This  difference  of  pressure  is  called  the  draft. 

As  a  matter  of  fact,  a  slight  difference  of  pressure  is 
required  to  cause  the  flow  of  gases  through  any  part  of  the 
boiler  and  the  drop  in  pressure  through  the  fuel  bed  is  only 
part  of  the  total  draft  required. 

The  draft  may  be  created  in  two  distinctly  different 
ways.  It  may  be  caused  by  a  chimney  or  stack,  and  is 
then  known  as  natural  draft,  or  it  may  be  produced  by  fans 
or  blowers,  in  which  case  it  is  called  mechanical  draft. 

(a)  Chimneys  or  Stacks.  Stacks  are  practically  always 
used  in  small  plants  because  of  the  simplicity  resulting  from 
their  use  and  because  the  interest  on  the  investment  com- 
pares favorably  with  interest  on  investment  plus  cost  of 
operation  for  mechanical  draft.  In  large  plants  fitted  with 
some  types  of  mechanical  stokers,  or  where  fuel  is  to  be 
burned  at  a  high  rate,  or  where  the  flue  gases  are  to  be  used 
for  heating  feed  water,  mechanical  draft  is  generally  installed. 
A  stack  of  some  sort  is  necessary  even  though  mechanical 
draft  be  used,  because  the  products  of  combustion  must  be 
discharged  at  a  sufficient  elevation  to  prevent  their  being 
a  public  nuisance. 

A  chimney  serves  to  carry  away  the  hot  products  of 
combustion  arid  when  in  operation  is  filled  with  a  column  of 
gases  with  higher  average  temperature  than  that  of  the 
surrounding  air.  As  a  result  the  density  of  gases  within 
the  stack  is  less  than  the  density  of  the  outer  air  and  the 
gas  pressure  at  the  bottom  of  the  structure  is  less  inside 
the  stack  than  it  is  outside.  If  an  opening  is  made  at  this 
point,  the  external  air  will  therefore  flow  in.  By  arranging 


STEAM  BOILERS 


371 


the  apparatus  as  shown  in  Fig.  227,  the  temperature 
of  the  air  flowing  into  the  bottom  of  the  stack  is  raised 
as  it  passes  through  the  furnace  and  the  flow  is  thus  made 
continuous. 

The  height  of  the  chimney  determines  the  draft  created 
by  it  with  flue  gases  of  a  given  temperature,  and,  with  any 
given  height,  the  area  determines  the  quantity  of  gas  which 


Atmospheric  Pressure 
i  mount  caused  by 
low  dtnbity  of  hot  gases 
in  fatatk. 


FIG.  227. — Diagrammatic  Arrangement  of  Stack. 

can  be  carried  off  in  a  given  time.  The  proportions  of 
chimneys  can  be  determined  from  rational  formulas  based 
on  theoretical  considerations,  but  it  is  necessary  to  assume 
values  for  a  number  of  constants  and  a  proper  choice  de- 
pends largely  upon  experience. 

As  a  result,  all  but  the  more  important  chimneys  are 
generally  designed  on  an  empirical  basis  and  many  formulas 
have  been  developed  for  this  purpose.  One  of  the  most 
common  methods  of  design  is  to  choose  the  height  in  accord- 


372  STEAM  POWER 

ance  with  the  values  given  in  Table  XVI,  and  then  to 
determine  the  sectional  area  according  to  an  empirical 
assumption  or  formula. 

TABLE  XVI 

COMMON  HEIGHTS  OF  CHIMNEYS 

{Applicable  to  plants  smaller  than  about  700  H.P.  Larger  installations  should 
have  stacks  of  from  150  to  175  feet  in  height  unless  local  conditions  call  for 
greater  height.) 


Character  of  Fuel. 

Height  above  Grate  in  Feet. 

Free-burning  bituminous                        .    . 

80 

Anthracite,  medium  and  large  sizes  
Slow-burning  bituminous  

100 
120 

Anthracite  pea  size.                

130 

Anthracite,  buckwheat  sizes  

150 

Thus,  some  designers  simply  assume  the  sectional  area 
at  the  top  of  the  stack  equal  to  about  one-ninth  of  the  grate 
area  for  anthracite  coal  and  equal  to  about  one-seventh  of 
the  grate  area  for  bituminous  coal.  Others  use  a  formula 
developed  by  William  Kent,  which  is  based  upon  the 
assumption  that  the  stack  should  be  large  enough  to  carry 
away  all  the  gases  resulting  from  the  combustion  of  5  Ibs. 
of  coal  per  rated  boiler  horse-power  per  hour.  This  formula 
gives  the  boiler  horse-power  which  the  stack  can  serve 
and  is 

HJ\  =  3.33(A-0.6VZ)V#,    .     .     .     (102) 
in  which 

H. P.  =  Rated  boiler  horse-power; 

A  =  Internal  sectional  area  in  feet  of  circular  or  square 

chimney; 
H  =  Height  above  grate  in  feet. 

(6)  Mechanical  Draft.  Fans  can  be  so  used  as  to  force 
air  into  the  ash  pit,  that  is,  to  raise  the  pressure  on  the 


STEAM  BOILERS  373 

entering  side  of  the  fire.  In  such  cases  the  equipment  is 
said  to  give  forced  draft.  Or  fans  may  be  installed  at  the 
discharge  end  of  the  flues  and  may  "draw  "  the  gases 
through  the  boiler  by  lowering  the  pressure  within  to  a  value 
below  that  of  the  external  atmosphere.  Such  an  instal- 
lation is  said  to  give  induced  draft. 

Forced  draft  suffers  from  the  disadvantage  that  the 
pressure  within  the  furnace  is  greater  than  atmospheric  and 
hot  gases  may  therefore  be  blown  out  when  the  fire 
door  is  opened.  On  the  other  hand,  the  fan  handles  only 
cool  air  instead  of  hot  products  of  combustion  as  in 
the  case  of  induced  draft  and  its  useful  life  is  therefore 
much  longer.  Forced  draft  is  much  more  common  than 
induced  draft. 

Several  arrangements  giving  balanced  draft  have  been 
developed.  With  such  apparatus  a  pressure  equal  to  atmos- 
pheric is  maintained  above  the  fuel  bed  and  no  hot  gases 
are  blown  out  through  the  firing  door. 

PROBLEMS 

1.  The  equivalent  evaporation  of  a  boiler  during  a  certain 
test  was  3450  Ibs.  per  hour.     What  boiler  horse-power  was  de- 
veloped? 

2.  A   water-tube   boiler   with   5000   sq.ft.   of   heating   surface 
and  rated  in  the  ordinary  way  gave  an  equivalent  evaporation 
of  25,875  Ibs.  per  hour.     At  what  per  cent  of  rating  was  the 
boiler  operating? 

3.  A  certain  boiler  produced  3500  Ibs.  of  dry  steam  in  one 
hour  from  feed  water  at  a  temperature  of  50°  F.     The  steam 
pressure  was  200  Ibs.   per  square  inch   gauge.     What  was  the 
equivalent  evaporation? 

4.  A  boiler  receiving  water  at  a  temperature  of  250°  F.  con- 
verts it  into  superheated  steam  at  a  pressure  of  210  Ibs.  per  square 
inch  gauge  and  a  temperature  of  580°  F.     The  boiler  produces 
26,000  Ibs.  of  steam  per  hour.     What  is  the  equivalent  evaporation 
if  the  boiler  is  given  credit  for  all  the  heat  given  the  material 
passing  through  it?     What  boiler  horse-power  is  developed! 

5.  A  boiler  produces  7.5  Ibs.  of  dry  steam  per  pound  of  coal 
fired.    The  feed-water  temperature  is  80°  F.  and  the  steam  pres- 


374  STEAM  POWER 

sure  is  125  Ibs.  per  square  inch  absolute.     What  is  the  equivalent 
evaporation  per  pound  of  coal? 

6.  A  boiler  is  supplied  with  coal  which  has  a  calorific  value 
of  13,520  B.t.u.  per  pound.  It  produces  8  Ibs.  of  dry  saturated 
steam  at  a  pressure  of  150  Ibs.  per  square  inch  gauge  per  pound 
of  coal.  The  feed-water  temperature  is  70°  F.  What  is  the 
efficiency  of  the  outfit? 


CHAPTER  XVIII 
RECOVERY  OF  WASTE  HEAT 

156.  Waste  Heat  in  Steam  Plant.  There  are  two  great 
heat  wastes  in  the  steam  plant — the  waste  in  the  hot  gases 
going  up  the  stack  and  the  waste  in  exhaust  steam.  The 
magnitude  of  the  stack  loss  can  best  be  appreciated  by 
determining  an  approximate  value  for  assumed  conditions. 
For  this  purpose  assume  the  fuel  to  be  pure  carbon,  the 
excess  coefficient  1.5,  average  atmospheric  temperature  60° 
F.,  average  stack  temperature  600°  F.,  and  no  moisture  in 
the  air.  The  specific  heat  of  the  flue  gases  may  be  taken 
as  constant  and  equal  to  0.24. 

With  an  excess  coefficient  of  1.5,  the  total  weight  of 
flue  gas  per  pound  of  carbon  burned  would  be  about  18.4 
Ibs.  and  the  heat  carried  up  the  stack  figured  above  room 
temperature  would  be 

Stack  loss  =  18.4X0.24  (600-60). 

=  2380  B.t.u.  per  pound  of  C  burned  (approx.) 

With  a  calorific  value  of  14,600  B.t.u.  per  pound  of  carbon 
this  loss  would  be  equivalent  to  a  little  over  16  per  cent 
of  the  total  heat  in  the  fuel. 

It  would  be  more  correct  to  use  the  temperature  of 
the  steam  in  the  boiler  instead  of  room  temperature,  because 
the  lowest  temperature  theoretically  attainable  by  gases 
passing  through  a  boiler  would  be  equal  to  that  of  the  steam 
and  water  on  the  other  side  of  the  heating  surface.  Under 
ordinary  conditions  of  operation,  this  method  of  figuring 
would  give  a  theoretically  avoidable  stack  loss  equal  to 
about  50  per  cent  of  the  figure  obtained  above. 

375 


376  STEAM  POWER 

The  magnitude  of  the  exhaust  loss  can  be  similarly 
approximated.  Assume  for  this  purpose  an  engine  receiving 
dry  saturated  steam  at  115  Ibs.  absolute  per  square  inch  and 
exhausting  it  with  a  quality  of  90  per  cent  at  a  pressure  of 
15  Ibs.  absolute  per  square  inch. 

The  heat  above  32°  in  the  entering  steam  is  1188.8  B.t.u. 
per  pound  and  the  heat  exhausted  per  pound  is  1053.7. 
The  heat  in  the  exhaust  represents  therefore  about  89  per 
cent  of  all  the  heat  supplied  when  calculations  are  made 
above  a  temperature  of  32°  F.  If  a  feed-water  tempera- 
ture of  60°  be  assumed  and  heat  quantities  be  figured 
above  that  datum  the  results  are  practically  the  same. 

There  are  always  numerous  pieces  of  auxiliary  apparatus 
in  steam  plants  such  as  boiler-feed  pumps,  circulating  pumps, 
vacuum  pumps,  etc.  These  are  often  steam  driven  and  are 
generally  very  uneconomical  in  the  use  of  heat,  so  that  they 
throw  away  in  their  exhaust  steam  large  quantities  of  heat 
originally  transferred  from  fuel  to  water  and  steam  in  the 
boiler. 

157.  Utilization  of  Exhaust  for  Heating  Buildings.  It 
often  happens  that  steam-power  plants  are  located  within 
or  in  the  neighborhood  of  buildings  requiring  artificial  heat 
during  part  of  the  year.  In  such  cases  the  exhaust  steam 
from  main  and  auxiliary  engines  can  generally  be  advan- 
tageously used  for  this  purpose.  Under  particularly  favor- 
able circumstances,  the  weight  of  steam  required  by  the 
plant  may  equal  approximately  that  required  for  heating, 
and  the  greater  part  of  the  exhaust  could  then  be  turned 
directly  into  the  heating  system. 

The  engines  in  plants  of  this  character  may  be  regarded 
as  reducing  valves  for  the  heating  system,  receiving  steam 
at  high  pressure  and  reducing  the  pressure  to  the  value  best 
adapted  to  the  heating  system  installed.  If  the  com- 
paratively small  losses  arising  from  radiation  from  the 
engine,  from  friction  and  from  the  presence  of  hot  water 
in  the  exhaust  be  neglected,  all  heat  received  by  the  engine 


RECOVERY  OF  WASTE  HEAT  377 

and  not  turned  into  useful  mechanical  energy  is  made  use 
of  in  the  heating  system.  The  engine  may  therefore  be 
very  uneconomical  in  the  use  of  steam  and  still  not  cause 
a  waste  of  fuel,  provided  always  that  the  heating  system 
can  absorb  all  heat  exhausted. 

Since  the  demands  of  a  heating  system  vary  from  day 
to  day  and  since  there  is  generally  no  demand  for  heat 
during  several  months  of  each  year,  it  follows  that  a  high 
degree  of  skill  is  necessary  in  choosing  the  character  of  the 
apparatus  installed.  A  compromise  is  generally  made 
between  the  cheap  and  uneconomical  engine  allowable  during 
the  coldest  months  and  the  more  expensive  and  more 
efficient  engine  desirable  when  no  heating  is  to  be  done. 

There  are  other  cases  of  somewhat  similar  character. 
In  many  industries  use  can  be  made  of  exhaust  steam  for 
the  heating  of  evaporating  pans,  dye  vats,  kilns  and  other 
apparatus.  Steam  plants  of  an  uneconomical  character  may 
be  very  economical  financially  in  connection  with  such 
industries  if  all  or  nearly  all  of  the  heat  in  the  exhaust  can 
be  utilized  industrially. 

158.  Feed-water  Heating.  An  examination  of  the  steam 
table  will  show  that  the  total  heat  above  32°  F.  per  pound 
of  steam  varies  between  1180  and  1200  B.t.u.  for  such  pres- 
sures as  are  commonly  used  in  boilers.  The  average  tem- 
perature of  water  as  it  occurs  on  the  surface  of  the  earth 
is  probably  somewhere  in  the  neighborhood  of  60°,  so  that 
the  heat  above  32°  per  pound  would  roughly  average  27 
B.t.u.  A  boiler  receiving  water  at  60°  and  converting  it 
into  steam  at  any  of  the  ordinary  pressures  must  therefore 
supply  over  1100  B.t.u.  per  pound  of  water. 

This  immediately  suggests  a  use  for  heat  in  exhaust 
steam.  Steam  exhausted  into  very  low  vacuums  has  a 
temperature  only  10°  to  30°  higher  than  the  assumed  average 
natural  feed  temperature,  but  steam  exhausted  at  atmos- 
pheric pressure  has  a  temperature  of  212°  F.  and  could 
therefore  impart  large  quantities  of  heat  to  water  at  60°  F. 


378  STEAM  POWER 

Since  the  boiler  must  supply  over  1100  B.t.u.  per  pound 
of  steam  made,  raising  the  feed  temperature  about  11°  or 
12°  should  effect  a  saving  of  about  1  per  cent  in  fuel  con- 
sumption. By  raising  the  temperature  from  60°  to  212° 
there  should  therefore  result  a  saving  of  approximately  13 
to  14  per  cent. 

Other  advantages  which  would  accrue  from  this  pre- 
liminary heating  of  the  feed  water  would  be  (1)  the  deposit, 
outside  of  the  boiler,  of  a  large  amount  of  the  solid  matter 
carried  by  the  water,  (2)  the  use  of  fewer  or  smaller  boilers, 
and  (3)  the  reduction  of  the  strains  which  occur  in  the  metal 
of  some  designs  when  very  cold  feed  water  is  used. 

Exhaust  steam  feed-water  heaters  are  divided  into  two 
types,  open  and  closed  heaters.  In  open  heaters  the  steam 
and  feed  water  are  brought  into  intimate  contact  in  the 
form  of  jets,  sheets  and  sprays  within  a  vessel  of  appropriate 
size  and  shape.  They  are  often  called  contact  heaters. 
When  the  exhaust  steam  comes  from  reciprocating  engines 
it  always  carries  in  suspension  some  of  the  oil  used  for 
lubricating  the  engine  cylinders.  If  allowed  to  enter  the 
heater,  this  oil  would  mix  with  the  feed  water  and  eventually 
reach  the  boilers,  where  it  might  cause  serious  damage  by 
depositing  upon  heating  surfaces  exposed  to  the  fire  or  to 
very  hot  gases.  Such  heaters  are  therefore  always  fitted 
with  oil  or  grease  extractors  when  used  with  reciprocating 
units.  When  receiving  the  exhaust  from  turbines,  oil  ex- 
tractors are  not  necessary,  as  no  lubricant  is  used  within 
the  steam  spaces  of  such  units. 

Closed  heaters  consist  of  tubes  or  coils  enclosed  within 
a  metal  vessel.  One  medium  passes  through  the  tubes 
and  the  other  over  their  outer  surfaces.  Such  heaters  are 
therefore  often  called  non-contact  heaters. 

As  oil  is  a  poor  conductor  of  heat,  the  exhaust  steam 
from  reciprocating  units  should  be  passed  through  an  oil 
extractor  before  entering  a  closed  heater  in  order  that  the 
heating  surfaces  may  be  used  to  the  best  advantage. 


RECOVERY  OF  WASTE  HEAT  379 

Exhaust  steam  feed-water  heaters  are  often  divided  into 
primary  and  secondary  heaters.  This  distinction  has  nothing 
to  do  with  structure,  being  based  entirely  on  position  and 
temperature.  Thus  there  may  be  available  exhaust  steam 
at  a  pressure  below  atmospheric,  as  from  condensing  main 
units,  and  exhaust  steam  at  atmospheric  pressure  from  non- 
condensing  auxiliaries.  The  lower  pressure  steam  could  be 
used  to  heat  the  feed  water  in  a  primary  heater  and  the 
higher  pressure  steam  could  then  raise  its  temperature  still 
further  in  a  second  or  secondary  heater. 

The  other  great  waste,  that  in  the  stack  gases,  can  also 
be  partly  eliminated  by  using  some  of  it  to  heat  the  feed 
water.  As  the  highest  steam  temperature  ordinarily  avail- 
able in  the  exhaust  system  is  about  212°  F.,  and  as  the 
products  of  combustion  leaving  the  boilers  generally  have 
temperatures  in  the  neighborhood  of  600°  to  700°  F.,  it 
is  evident  that  on  a  basis  of  temperature  the  hot  gases 
have  a  decided  advantage  as  a  heating  medium.  On  the 
other  hand,  the  specific  heat  of  the  hot  gases  is  low,  while 
exhaust  steam  can  give  up  all  of  its  latent  heat  with  no 
change  in  temperature,  so  that  on  a  basis  of  heat  avail- 
able for  transmission  to  the  water,  the  steam  has  the 
advantage. 

The  waste  heat  in  the  flue  gases  is  used  for  feed- water 
heating  in  devices  known  as  economizers.  These  generally 
consist  of  groups  of  tubes,  joined  at  their  ends  by  headers 
and  standing  vertically  within  sheet-metal  flues  leading 
from  the  gas  passages  of  the  boilers  to  the  chimney.  The 
water  to  be  heated  is  pumped  through  the  tubes  on  its  way 
to  the  boilers  and  the  hot  gases  flow  over  the  tubes  on  their 
way  to  the  chimney.  Mechanically  operated  scrapers  are 
arranged  to  travel  up  and  down  the  tubes  at  intervals  and 
keep  their  external  surfaces  free  of  soot  and  dust,  which 
would  seriously  reduce  their  ability  to  transmit  heat. 

The  feed  water  supplied  the  economizer  is  generally 
first  heated  in  an  exhaust  steam  heater  and  arrives  at  the 


380  STEAM  POWER 

economizer  with  temperatures  between  about  120°  and 
200°  F.,  depending  upon  the  kind  of  heaters  used  and  upon 
the  relative  quantities  of  exhaust  steam  and  feed  water. 
The  economizer  discharges  the  water  to  the  boiler  at  tem- 
peratures which  generally  run  from  about  210°  to  300°  F., 
depending  upon  the  amount  of  preliminary  heating,  the 
extent  of  economizer  surface  and  a  number  of  other  variables. 
The  gases  which  have  passed  through  an  economizer 
often  have  temperatures  as  low  as  250  to  350°  F.,  which  is 
generally  too  low  a  value  to  give  good  chimney  draft. 
Plants  making  extensive  use  of  economizers  are  therefore 
generally  fitted  with  some  form  of  mechanical  draft. 

PROBLEMS 

1.  Determine  the  heat  lost  in  the  chimney  gases  per  pound 
of  coal  in  a  plant  operating  under  the  following  conditions,  and 
express  the  loss  as  a  percentage  of  the  heat  value  of  the  coal. 
The  coal  has  a  calorific  value  of  14,000  B.t.u.  per  pound;    the 
temperature  of  the  gases  leaving  the  boiler  is  570°  F.;   20  Ibs.  of 
gas  result  from  each  pound  of  coal  burned;    the  mean  value  of 
the  specific  heat  of  the  gases  is  0.245;    and  the  temperature  of 
the  air  entering  the  furnace  is  75°  F. 

2.  Determine  the  quantity  of  heat  which  could  be  obtained 
from  the  gases  of  Prob.  1  by  using  an  economizer  to  reduce  their 
temperature  to  250°  F.    What  percentage  of  the  heat  value  of 
a  pound  of  coal  does  this  saving  represent? 

3.  The  boilers  of  a  certain  plant  produce  100,000  pounds  of 
steam  per  hour  when  the  plant  is  operating  at  full  load.     The 
steam-driven  auxiliaries  consume  10%  of  this  steam.     Steam  is 
generated  at  a  pressure  of  175  Ibs.   per  square  inch  gauge,  and 
is  superheated  150°  F.     The  main  units  operate  condensing  and 
the  condensate  leaves  the  condensers  at  a  temperature  of  75°  F. 
The  auxiliaries  operate  non-condensing  and  exhaust  their  steam 
at  atmospheric  pressure  and  with  a  quality  of  92%.     The  coal 
used  has  a  calorific  value  of  13,850  B.t.u.    The  boiler  efficiency 
/Heat  given  water  and  steamX  ,    „„... 

I     ° . _,. ,.    I      -jo       /  ^^Y 

\       Heat  in  fuel  supplied      / 

(a)  Determine  the  amount  of  coal  which  would  have  to  be 
burned  per  hour  if  the  steam  exhausted  from  the  auxiliaries  were 
thrown  away  and  make-up  water  at  a  temperature  of  50°  F.  were 


RECOVERY  OF  WASTE  HEAT  381 

used  in  its  place.  The  condensate  from  the  condensers  of  the 
main  unit  is  assumed  to  be  returned  to  the  boiler  after  being 
mixed  with  the  make-up  water. 

(6)  Determine  the  amount  of  coal  which  would  have  to  be 
burned  per  hour  if  the  auxiliary  exhaust  were  used  to  heat  the  con- 
densate from  the  main  units  in  an  open  heater  and  if  the  operation 
of  the  plant  were  so  perfect  that  no  make-up  water  had  to  be 
added. 


CHAPTER  XIX 
BOILER-FEED  PUMPS  AND  OTHER  AUXILIARIES 

159.  Boiler-feed  Pumps.  The  pumps  used  for  forcing 
the  feed  water  into  boilers  may  be  of  reciprocating  or 
centrifugal  construction  and  may  be  driven  by  reciprocating 
steam  cylinders,  by  small  steam  turbines  or  by  electric 
motors. 

Steam-driven  pumps  are  very  wasteful,  often  using  over 
100  Ibs.  of  steam  per  horse-power  hour.  It  would  therefore 
seem  more  economical!^  use  motor-driven  pumps  in  electric- 
power  stations,  as  the  large  power  units  will  generate  electric 
power  with  a  consumption  of  from  10  to  25  Ibs.  of  steam  per 
horse-power  hour  and  the  motor  efficiency  will  generally  be 
over  80  per  cent.  There  is,  however,  another  point  which 
must  be  considered.  The  exhaust  steam  from  small  engines 
operating  boiler-feed  pumps  can  be  used  for  heating  the 
feed  water  as  described  in  the  last  chapter,  and  thus  the  poor 
economy  of  these  units  is  of  little  significance ;  practically 
all  heat  exhausted  can  be  returned  to  the  boiler  in  the 
boiler  feed  if  desirable.  As  a  result  of  this  considera- 
tion, coupled  with  others  of  less  importance,  nearly  all 
boiler-feed  pumps  and  other  similar  auxiliaries  are  steam 
driven  unless  there  are  so  many  that  there  would  be 
more  exhaust  steam  than  could  be  absorbed  by  the  feed 
water. 

There  is  at  present  a  marked  tendency  toward  the  use 
of  turbine-driven,  centrifugal  pumps  for  boiler  feeding,  in 
place  of  those  driven  by  reciprocating  steam  units.  The 
turbine  type  has  several  advantages,  the  more  important 
being: 

382 


BOILER-FEED  PUMPS  AND  OTHER  AUXILIARIES   383 

(1)  No  oil  in  exhaust  steam,  so  that  latter  is  well  adapted 
to  use  in  all  forms  of  feed- water  heaters; 

(2)  Higher  speed  because  of  continuous  flow  of  water 
and  continuous  rotation  of  mechanical  parts,  thus  making 
possible  great  decrease  in  size  for  a  given  amount  of  work, 
and 

(3)  Better  pump  characteristics  for  this  sort  of  work. 

The  Duplex  Steam  Pump.  The  great  majority  of  re- 
ciprocating steam  pumps  used  for  boiler-feed  purposes  are 
of  the  duplex  pattern,  one  design  of  which  is  shown  in  Figs. 


Steam  End 


Water  End 


FIG.  228.— Duplex  Steam  Pump. 

228  and  229.  Two  steam  cylinders  are  arranged  side  by 
side,  their  piston  rods  extending  into  similarly  arranged 
water  cylinders  and  carrying  water  plungers  or  pistons  as 
shown  in  Fig.  229.  As  there  is  no  rotating  shaft  in  a  pump 
of  this  kind,  the  steam  valves  cannot  be  operated  by  eccen- 
trics as  is  common  with  steam  engines.  For  the  purpose 
of  operating  these  valves,  bell  cranks,  pivoted  near  the 
center  of  length  of  the  pump,  are  provided.  These  are 
arranged  so  that  the  long  arm  of  one  bell  crank  engages  a 
collar  on  the  piston  rod  of  one  steam  cylinder  and  the  short 
arm  operates  the  valve  gear  of  the  other  steam  cylinder. 
The  motion  of  the  valve  of  one  cylinder  is  therefore  derived 


384 


STEAM  POWER 


from  the  piston  motion  of  the  other  cylinder.  The  steam 
pistons  are  practically  180°  out  of  phase,  one  moving  out 
while  the  other  moves  in. 

Practically  no  expansion  of  the  steam  is  obtained  in  the 
cylinders  of  pumps  of  this  type.  They  operate  on  the 
rectangular  cycle  described  in  an  earlier  chapter  and  are 
correspondingly  wasteful  in  their  use  of  steam. 


Slide  Valves 


Steam  End 


Water  End 

FIG.  229.— Duplex  Steam  Pump. 


160.  The  Steam  Injector.  On  steam  locomotives  and 
in  other  portable  steam  plants,  as  well  as  in  many  small 
stationary  plants,  a  device  known  as  a  steam  injector  is  used, 
instead  of  a  pump,  for  forcing  feed  water  into  the  boiler. 
A  simple  form  of  steam  injector  is  shown  semi-diagram- 
matically  in  Fig.  230. 

Steam  from  the  boiler  flows  through  the  steam  nozzle 
and  expands  from  boiler  pressure  to  a  very  low  pressure, 
thus  acquiring  a  high  velocity  at  the  expense  of  the  heat 


BOILER-FEED  PUMPS  AND  OTHER  AUXILIARIES    385 

energy  which  it  brings  from  the  boiler.     At  the  end  of  the 
nozzle  it  mixes  with  water  and  imparts  to  that  water  some 


of  its  kinetic  energy,  so  that  the  mixture  moves  into  the 
small  end  of  the  delivery  tube  with  a  high  velocity.  By 
the  time  it  has  reached  that  point,  practically  all  the  steam 
has  been  condensed,  and,  as  the  sectional  area  of  the  delivery 


386  STEAM  POWER 

tube  increases,  the  velocity  of  the  liquid  decreases  with  a 
corresponding  increase  in  pressure  according  to  Bernoulli's 
theorem.  In  properly  designed  apparatus,  the  resultant 
pressure  is  great  enough  to  force  the  mixture  of  water  and 
condensed  steam  into  the  boiler  against  boiler  pressure. 

The  space  at  the  end  of  the  steam  nozzle  is  maintained 
at  a  low  temperature  by  the  feed  water  flowing  through 
it  and  the  pressure  of  the  steam  is  therefore  very  low  at  this 
point,  being  less  than  atmospheric  in  most  cases.  Atmos- 
pheric pressure  is  therefore  able  to  force  water  up  the  suction 
pipe  if  the  "  lift  "  is  not  too  great,  and  when  once  started 
such  a  device  can  therefore  "  raise  "  its  own  water  as  well 
as  delivering  it  against  pressure. 

It  is  interesting  to  note  that  the  efficiency  of  this  appa- 
ratus is  almost  100  per  cent  on  a  heat  basis.  All  heat  not 
radiated  from  the  apparatus  is  returned  to  the  boiler  in  the 
mixture  of  condensed  steam  and  feed  water  and,  as  the 
external  surface  is  very  small,  very  little  heat  is  lost  by. 
radiation. 

161.  Separators.  Two  kinds  of  separators  are  used  in 
steam  plants:  (a)  the  oil  separators  already  referred  to  for 
separating  oil  from  exhaust  steam,  and  (6)  steam  separators, 
which  separate  water  from  steam. 

As  it  is  impossible  entirely  to  prevent  radiation  from 
steam  pipes,  it  follows  that  condensation  will  occur  in  any 
pipe  line  which  carries  saturated  steam.  Water  is  also 
formed  in  the  cylinders  of  reciprocating  engines  not  supplied 
with  very  highly  superheated  steam,  and  much  of  it  is 
generally  present  in  the  exhaust  of  the  high  and  inter- 
mediate cylinders  of  multiple-expansion  engines. 

A  small  amount  of  water  can  be  passed  through  the 
cylinder  of  a  reciprocating  engine  without  mechanical 
damage,  but  it  probably  causes  a  loss  of  heat  by  cling- 
ing to  the  walls  and  assisting  in  the  heat  interchanges 
which  always  occur.  Large  quantities  of  water  are  apt 
to  cause  mechanical  damage,  as  water  is  inelastic,  and  if 


BOILER-FEED  PUMPS  AND  OTHER  AUXILIARIES    387 


Sieves 


/Jacket  of  Insulating 
Material  to  Decreasa 
Radiation  Loss. 


Drain-) 


FIG.  231. — Steam  Separator. 


388  STEAM  POWER 

more  of  it  is  trapped  in  a  cylinder  end  than  can  be  con- 
tained in  the  clearance,  something  must  give  way  when 
the  piston  reaches  the  end  of  its  stroke. 

It  is  customary  to  separate  as  much  as  possible  of  the 
water  of  condensation  before  admitting  steam  to  the 
cylinder.  The  separators  used  are  built  in  many  different 
shapes  and  types,  but  practically  all  depend  upon  two 
principles.  These  are: 

(1)  Water  is  much  more  dense  than  steam,  and  if  a 
stream  of  a  mixture  of  water  and  steam  be  made  to  travel 
in  a  curve,  the  water  will  therefore  collect  at  the  outside 
of  the  curve,  and 

(2)  Water  brought  into  violent  contact  with  metallic 
surfaces  "  wets  "  them  and  has  a  tendency  to  adhere  thereto. 

In  steam  separators  the  stream  of  mixture  is  therefore 
made  to  change  its  direction  of  flow  suddenly  and  to  impinge 
upon  baffles  in  such  a  way  that  the  greater  part  of  the 
liquid  is  caught  and  drained  off. 

One  form  of  separator  is  shown  in  Fig.  231.  The  mixture 
impinges  on  sieves  in  the  first  part  of  its  passage  through 
the  separator,  part  of  the  water  passing  through  the  open- 
ings and  draining  to  the  reservoir  at  the  bottom  of  the 
device.  Ridges  and  troughs  catch  all  water  separated  and 
guide  it  to  drains  leading  to  the  reservoir,  so  that  no  water 
which  is  once  deposited  is  again  picked  up  by  steam. 

Another  form  of  separator  is  illustrated  in  Fig.  232. 
The  steam  impinges  upon  the  inverted  V-shaped  casting 
and  water  caught  on  the  projecting  ridges  drains  toward 
the  sides  and  then  downward  into  the  receiver,  while  the 
steam  passes  on  as  shown. 

162.  Steam  Traps.  In  the  separators  just  described, 
there  is  a  constant  accumulation  of  water  which  must  be 
periodically  drained  off  if  the  entire  device  is  not  to  fill 
up  and  become  inoperative.  Similarly  there  is  a  constant 
accumulation  of  liquid  in  steam  jackets,  in  receivers  of 
multi-expansion  engines  and  in  low  points  in  steam  lines. 


BOILER-FEED  PUMPS  AND  OTHER  AUXILIARIES   389 


To  drain  all  such  accumulations  by  hand  from  time  to 
time,  as  necessary,  would  be  both  time  consuming  and 
uncertain,  and  devices  known  as  traps  have  therefore  been 
developed  for  doing  this  automatically.  Traps  are  arranged 
to  collect  condensation  until  they  fill  to  a  predetermined 
point.  When  this  occurs  they  automatically  discharge  in 
such  a  way  as  to  prevent  the  escape  of  steam  and  are  then 
ready  to  receive  liquid  again. 

Traps  may  be  arranged  to  receive  condensation  from 
high  or  low-pressure  mains  or  from  spaces  in  which  a  vacuum 


(<*)  (*) 

FIG.  232. — A  Steam  Separator. 

is  maintained,  and  to  discharge  it  to  a  hot  well  or  similar 
receiver  or  even  into  the  boiler  itself. 

The  principles  upon  which  traps  work  are  very  nu- 
merous and  there  are  many  different  designs.  The  more 
common  either  make  use  of  the  weight  of  the  accumulating 
water  to  cause  the  trap  to  discharge,  or  they  make  use  of 
floats  resting  on  the  condensate  and  opening  the  discharge 
valve  when  a  certain  height  is  reached,  or  they  depend  upon 
expansion  and  contraction  of  certain  parts  when  exposed 
to  steam  of  high  temperature  and  cooler  condensate  respect- 
ively. 


390  STEAM  POWER 

163.  Steam  Piping.  There  is  a  great  deal  of  piping  of 
various  kinds  in  all  steam  plants  and  the  financial  success 
or  failure  of  a  plant  often  depends  upon  this  apparently 
insignificant  item.  It  is  beyond  the  limits  of  a  book  of  this 
scope  to  consider  the  many  different  forms  of  piping  and  the 
many  different  ways  in  which  apparatus  may  be  connected. 
This  is  a  study  in  itself  and  one  of  great  importance. 

It  should  be  noted,  however,  that  all  of  the  following 
points  must  be  kept  in  view  when  designing  and  installing 
piping  and  that  that  installation  which  most  nearly  meets 
all  these  requirements  may  be  regarded  as  the  best. 

(1)  The  various  lines  should  conduct  the  materials  flow- 
ing through  them  with  the  minimum  loss  of  pressure  and 
with  the  minimum  loss  (or  gain)  of  heat. 

(2)  The  pipe  lines  should  be  so  constructed  as  to  make 
failure  of  a  dangerous  sort,  from  expansion  and  contraction, 
water  hammer  and  such,  most  unlikely  if  not  impossible. 

(3)  All  connections  should  be  so  made  that  the  careless 
manipulation  of  valves  cannot  cause  an  accident. 

(4)  The  number  of  flange  and  screw  connections  and  the 
number   of  valves  and  fittings  should   be  reduced  to  the 
minimum,  as  they  are  often   sources  of  weakness  and  are 
always  costly. 

(5)  The  entire  layout  should  be  so  arranged  that  inter- 
ruption of  service  because  of  pipe,  or  valve,  failure  is  (as 
nearly  as  possible)  impossible. 

(6)  The  cost  of  the  system  should  be  as  small  as  it  can 
be  made,  consistent  with  the  other  requirements. 

It  is  almost  unnecessary  to  say  that  all  of  these  desirable 
ends  are  never  attained  in  any  plant.  A  compromise  must 
always  be  made  in  order  to  bring  the  cost  within  reasonable 
limits,  but  most  of  the  recent  installations  show  a  tendency 
toward  better  design  in  this  part  of  the  plant  and  a  con- 
sideration of  reliability  and  safety  far  in  excess  of  what  was 
formerly  customary. 


TABLES 


392 


STEAM  POWER 


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SATURATED  STEAM  TABLE 


393 


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394 


STEAM  POWER 


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SATURATED  STEAM  TAJJLE 


395 


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t^.  os  i—  t  co  to 

00  00 


396 


STEAM  POWER 


10  o  in  o  10      o  10  o  10  o 

OiHiHCNOl         WCO-^T^IO 
tH  TH          rHiHrHrHT-4 


10  o  10  o  10 

10  CO  «O  t-  t- 


CM  T-H  OS  CM  CM        O  "*  CO  iO  CM 

iccor-  if-Hi—  i      CM  co  >o  i^  o 

OS  00  t^  CO  CO 


TF  Tt<  CO  CO  CO        COCOCOCOCO 


<M»O<MO  Oi  i—  i  CO  ^-H  00 

'^t^'-H^O  00  CO  l>-  (M  CO 

t^COCOiO  TjHTjncOCOiM 

f^i  ^)  ^^  ^^  ^^  ^^  ^  CO  O 


Oi  rtH  l^  C5  O5  CO  »O  (M  i>-  (N  1C  GO  O5  O5  O5 
OCi  CO  t^*  T™H  *O  O^  CO  t^*  CO  ^f  t^*  O  CO  CO  OS 
!>•  00  GO  O^  OS  OS  O  O  *-"*  ^  ^"^  C^l  ^1  0s!  C^ 


o  o  00  co     OCOCDO     cDcoo 


CO  CM  1>*  CO  OS 

osososoooo 

LQiOO'O'O 


OS  *O  CO  t^* 

coco»o^o 


5 


*a 

X  fl 
H 


CO  00  OS  O  i—  I        CO  "^  CO  t^  00        OS  O  CM  CN  CO 

^  ,_«'  ,-i  <M'  <M'      <M'  c<i  cq  CM'  oi 

oooooooooo      oooooooooo 


CM  co  co  co  co 
oooooooooo 


t^  1C  CM        O  t^  iO  CM  O        00  iO  CO  rH  OS 
O5OSOS        OSOOX'OOOO        t^t^l>t^cO 


cO'O'OiO 

oooooooo 


o  to  o  co  10 

CM  iO  OS  CM  *O 

CO  CO  CO  CO  CO 


CO  1>  CO  "*  CM        OS  CO  CM  I>  CM 


GO  rH  Tf  t^  O 

r-tcMCMCMCO 

COCOCOCOCO 


CM  O  00  O  CO 
COCOCO'*T}H 
COCOCOCOCO 


CM  O  00  CO  CO 

t^OOOOOsd 

OOOOOOOOOs 


O  CO  CM  00  ^ 

l-HrHCM<M'cO 

OSOSOSOSOS 


O  »O  O  **  OS 

TfHTt<lOlO»o' 

OSOSOSOSOS 


^  00  r-  (  CO  ^ 


rfi  CO  i—  1  GO  O 

l>  O  CO  ^O  00 
Tti  iO  »O  iO  iO 
COCOCOCOCO 


O  CO  O  »O  GO 


10  0  iO  0  10    0  10  O  10  O    10  0  10  O  10 

OY-HI-IC^O*      coeoTj<Tt«»o      10  «c>  «o  t>  t» 


COCOCOCOCO         COCOCOCOCO        COCOCOCOCO 

§'100^0      10  o  o  o  10      o 
OSOOrH        t-HCMCMCOCO        Ttn 


SATURATED  STEAM  TABLE 


397 


CO  GO  CO  CO  O  O  i— < 

COCOO^Ci  tO  to  l>  CO  to 

tO  Tt^  ^  CO  C^l  GO  tO  I~H  O-  ^f  C^  CO 

<M  <M  <M  <M  <M  rHrHrHOOOO 


rH  CO  rH  CO  rH 


O  GO  <M 
(M  rH  rH  O  O     CO  <N  CO  <N  CO  IO  , 

OOOOO       CT.  O  00  00  CO  TF 


00000        OOOOO 


CO  O5 

t^  C"l  O5  O 

C^  rH  (X)  t>.  O5 

tOtOtOtOtO        tOtOTt^cO' 


Tfi  T^H  to  COl>       CO  to 


CO  CO  CO  CO  CO 
00  GO  00  GO  00 


00  O  O 

GOOO  x  oc  t^  co 


1>  tO  CO  rH  Oi 

COCO  CO  CO  iO 


C  00 

<M  CO  rH  GO  t>» 

t^ 

co 


^  (M  O  t^OOC 

to 


00  00  Ci  O  (M 

O  00  CO  iO  CO 
00  00  00  00  00 


CO  CO  O 
CO  r—  <  CO  (N 

C<l  rH  00  CO 

oo  oo  t^  i> 


i  to  to 
COCO  CO  CO  CO 


^.rH  »OrH 


COCOt-t-GO  rH 

Oi  Oi  O5  Oi  C7i       O 


0  0  0  S 
<M  (N  (M  (N 


rH  rf  CO  00  Oi          rH  IO  GO  CO 
COtOI>a5rH  rH  t^  •<*  1>  tO  O  CO 


§uoo  to 
CO  Oi  O) 


cococococo      cocococo    •  +i 

«OO>OOtO        tOtOtOtOOO  ^ 

coi^i^oooo      coooooooci  O 

C^  (N  CO  -^  *    *  * 


• 


398 


STEAM  POWER 


PROPERTIES  OF  ONE  POUND  OF  SUPERHEATED  STEAM 

[Condensed  from  Marks  and  Davis's  STEAM  TABLES  AND  DIAGRAMS,  1909,    by 
permission  of  the  publishers,  Longmans,  Green  &  Co.] 

Sp.V.  =  specific   volume    in    cu.ft.;     AQ  =  B.t.u.    total   heat   above 
32°  F.;  A0  =  total  entropy  above  32°  F. 


Absolute 
Pressure. 
Lbs.  Sq.in. 

Degrees  of  Superheat. 

Sat.  Temp. 
0  F. 

0 

50 

100 

150 

200 

250 

300 

15      | 

(213)   } 

Sp.V. 
AQ 

A<£ 

26.27 
1150.7 
1  .  7549 

28.40 
1174.2 

1.7886 

30.46 
1197.6 
1.8199 

32.50 
1221.0 
1.8492 

34.53 
1244.4 

1.8768 

36.56 
1267.7 
1.9029 

38.58 
1291  .  1 
1.9276 

60      | 

(281)    ) 

Sp.V. 
AQ 
A0 

8.51 
1173.6 
1  .  6581 

9.19 
1198.8 
1  .  6909 

9.84 
1223  .  4 
1.7211 

10.48 
1247.7 
1.7491 

11.11 
1271.8 
1  .  7755 

11.74 
1295.8 
1.8002 

12.36 
1319.7 
1.8237 

100     j 

(327.  8)  | 

Sp.V. 
AQ 

A0 

4.43 
1186.3 
1.6020 

4.79 
1213.8 
1.6358 

5.14 
1239  .  7 
1.6658 

5.47 
1264.7 
1  .  6933 

5.80 
1289.4 

1.7188 

6.12 
1313.6 
1  .  7428 

6.44 
1337.8 
1  .  7656 

110     f 

(334.  8)  1 

Sp.V. 
AQ 

A0 

4.05 
1188.0 
1  .  5942 

4.38 
1215.9 
1  .  6282 

4.70 
1242.0 
1.6583 

5.01 
1267.1 
1.6857 

5.31 
1291.9 
1.7110 

5.61 
1316.2 
1  .  7350 

5.90 
1340.4 
1.7576 

120     f 

(341.  3)  1 

Sp.V. 
AQ 
A0 

3.73 
1189.6 
1.5873 

4.04 
1217.9 
1.6216 

4.33 
1244.1 
1.6517 

4.62 
1269.3 
1  .  6789 

4.89 
1294.1 
1  .  7041 

5.17 
1318.4 
1.7280 

5.44 
1342.7 
1.7505 

130     f 

(347.  4)  j 

Sp.V. 
AQ 
A<£ 

3.45 
1191.0 
1.5807 

3.74 
1219.7 
1.6153 

4.02 
1246.1 
1  .  6453 

4.28 
1271  A 
1.6724 

4.54 
1296.2 
1.6976 

4.  80 
1320.6 
1.7213 

5.05 
1344.9 
1  .  7437 

140    j 
(353.  1)| 

Sp.V. 
AQ 
Atf> 

3.22 
1192.2 
1.5747 

3.49 
1221  .4 
1  .  6096 

3.75 
1248.0 
1.6395 

4.00 
1273.3 
1.6666 

4.24 
1298.2 
1.6916 

4.48 
1322.6 
1.7152 

4.71 
1346.9 
1.7376 

150     f 

(358.  5)  | 

Sp.  V. 
AQ 

A0 

3.01 
1193.4 
1  .  5692 

3.27 
1223.0 
1  .  6043 

3.51 
1249  .  6 
1  .  6343 

3.75 
1275.1 
1.6612 

3.97 
1300.0 
1.6862 

4.19 
1324.5 
1.7097 

4.41 
1348.8 
1  .  7320 

SUPERHEATED  STEAM  TABLE 


399 


PROPERTIES  OF  ONE  POUND  OF  SUPERHEATED  STEAM 

(Continued) 


Absolute 
Pressure. 
Lbs.  Sq.in. 

Degrees  of  Superheat. 

250 

300 

0 

50 

100 

150 

200 

Sat.  Temp. 
0  F. 

160     { 
(363.6)} 

Sp.  V. 
AQ 

A</> 

2.83 
1194.5 
1.5693 

3.07 
1224.5 
1.5993 

3.30 
1251.3 
1  .  6292 

3.53 
1276.8 
1.6561 

3.74 
1301.7 
1  .  6810 

3.95 
1326.2 
1.7043 

4.15 
1350.6 
1.7266 

170     f 

(368.5)} 

Sp.  V. 
AQ 
A0 

2.68 
1195.4 
1  .  5590 

2.91 
1225.9 
1  .  5947 

3.12 
1252.8 
1.6246 

3.34 
1278.4 
1.6513 

3.54 
1303  .  3 
1.6762 

3.73 
1327.9 
1.6994 

3.92 
1352.3 
1.7217 

180     f 

(373.1)} 

Sp.  V. 
AQ 
A<£ 

2.53 
1196.4 
1.5543 

2.75 
1227.2 
1.5904 

2.96 
1254.3 
1.6201 

3.16 
1279.9 
1  .  6468 

3.35 
1304.8 
1.6716 

3.54 
1329.5 
1  .  6948 

3.72 
1353.9 
1.7169 

190     { 

(377.  6)  1 

Sp.  V. 
AQ 

A<£ 

2.41 
1197.3 
1.5498 

2.62 
1228.6 
1  .  5862 

2.81 
1255.7 
1.6159 

3.00 
1281.3 
1.6425 

3.19 
1306.3 
1.6627 

3.37 
1330.9 
1.6904 

3.55 
1355.5 
1.7124 

200     j 

(381.  9)  \ 

Sp.  V. 
AQ 
A<£ 

2.29 
1198.1 
1.5456 

2.49 
1229  .  8 
1.5823 

2.68 
1257.1 
1.6120 

2.86 
1282.6 
1  .  6385 

3.04 
1307.7 
1  .  6632 

3.21 
1332.4 
1  .  6862 

3.38 
1357.0 
1  .  7082 

300     f 

(417.5)} 

Sp.  V. 
AQ 

A</> 

1.55 
1204.1 
1.5129 

1.69 
1240.3 
1.5530 

1.83 
1268.2 
1.5824 

1.96 
1294.0 
1.^082 

2.09 
1319.3 
1.6323 

2.21 
1344.3 
1.6550 

2.33 
1369.2 
1.6765 

500     f 

(467.3)) 

Sp.  V. 
AQ 
A<£ 

0.93 
1210.0 
1.470 

1.03 
1256 
1.519 

1.11 
1285 
1.548 

1.22 
1311 
1.573 

1.31 
1337 
1.597 

1.39 
1362 
1.619 

1.47 

1388 
1.640 

INDEX 


PAGE 

Absolute  pressures 41 

Absolute  temperature  scale 12 

Action  of  steam,  in  cylinder 24 

on  impulse  blades  of  steam  turbine 234-236 

Adiabatic  expansion 58 

Advance  angle 167 

Advantages  of  condensing 251,  252 

Advantages,  relative,  of  contact  and  non-contact  condensers. .  274,  275 

Air,  excess,  combustion 286 

Advantages  and  disadvantages  of 311 

Analogy,  hydraulic 26 

Analyses  of  coal  (see  fuels) 299-301 

Purchase  of  coal  on  analysis 302 

Angle  of  advance 167 

Ash  in  coal 300 

Atmospheric  line  on  indicator  diagram 119 

Atoms 278 

Avogadro's  Law 281 

Babcock  &  Wilcox  superheater 367 

' '          "        "       water-tube  boiler 352-355 

Balanced  slide  valves 184 

Barometer,  conversion  of  readings  from  inches  mercury  to  pounds 

per  square  inch 255,  256 

Barometric  Condenser 261-266 

Baume  scale  to  express  gravity 303 

Bearings Ill,  112 

Bilgram  diagram 168-182 

Angularity  of  connecting  rod 179 

Diagram  for  both  cylinder  ends 177 

Exhaust  and  compression 175-177 

Indicator  diagram  from 180-183 

Piston  positions 177-182 

Blades,  impulse,  action  of  steam  on,  in  impulse  turbine 234-236 

401 


402  INDEX 


PAGE 


Boiler-feed  pumps  and  other  auxiliaries 382-390 

Boiler,  generation  of  steam  in 38,  39 

Boilers,  steam 305-374 

Circulation  in 341,  342 

Classification  according  to — 

(1)  form;    (2)  location  of  furnace;   (3)  use;    (4)  direction 
of  principal  axis;    (5)  relative  positions  of  water  and  hot 

gases 305,  306 

Draft  apparatus 370-373 

Chimneys  or  stacks 370-372 

Mechanical  draft 372,  373 

Effects  of  soot  and  scale 364,  365 

Efficiencies 363,  364 

Functions  of  parts 306-308 

Furnaces  and  combustion 308-311 

Hand  firing 311-315 

Mechanical  grates 315,  316 

Mechanical  stokers 317-335 

Rate  of  combustion 335-337 

Rating 358-362 

Boiler  horse-power 359 

Equivalent  evaporation 361 

Scale 365,  366 

Prevention  of 366 

Smoke  and  its  prevention .  .  . 316,  317 

Strength  and  safety 337-341 

Superheaters — 

Built  in 366,  367 

Separately  fired 366,  367 

Babcock  &  Wilcox 367 

Foster 369,  370 

Heine 367,  368 

Types  of  boilers 342-359 

Babcock  &  Wilcox,  water  tube ...;.. ...   352-355 

Continental 346-348 

Externally  fired,  return  tubular 349-352 

Heine  water-tube 355,  356 

Internally  fired,  tubular 342-345 

Locomotive • 346 

Scotch  marine 348,  349 

Sterling  water-tube 356-358 

Vertical  fire  tube 343-345 

Wickes  vertical  water-tube 358,  359 


INDEX  403 

PAGE 

British  Thermal  unit 3,  13 

Buildings,  heating  of,  by  exhaust  steam 376,  377 

Built-in  superheaters .   366,  367 

Calorific  value  of  coals — 

Dulong's  formula 301,  302 

Fuel  Calorimeter 302 

Calorific  value  of,  petroleum  oils 303,  304 

Calorimeter,  fuel 302 

Carbon,  combustion  of 279 

CO,  combustion  to 279-282 

CO.  combustion  to 282,  283 

CO  and  CO.,  conditions  determining  formation  of. .  .    284-286 

CO  to  CO2,  combustion  of 283,  284 

complete  combustion  of 308-310 

flue  gases  from  combustion  of 286,  287 

Card  factors  and  conventional  diagram 125-128 

Centigrade  scale 10,  11,  12 

Chain  grate  stokers .  . 318-322 

Chart — 

Mollier,  for  steam 230 

temperature-entropy,  for  steam 62-65 

Chimneys  or  stacks 370-372 

Circulation  in  boilers 341-342 

Classification  of  boilers,  according  to — 

(1)  form;   (2)  location  of  furnace;    (3)  use;   (4)  direction  of  prin- 
cipal axis;  (5)  relative  postion  of  water  and  hot  gases 305-374 

Classification  of  steam  engines 92,  93 

Clearance — steam  engine — 

mechanical  and  volumetric 84,  85 

Clearance  volume  determined  from  diagram 131,  132 

Closed  and  open  feed-water  heaters 377-380 

Coal-fuels 297-299 

Analyses  of — proximate  and  ultimate 299-301 

Purchase  of,  on  analysis 302 

Coefficient,  excess  in  combustion 286 

Combined  indicator  diagrams 155-158 

Combined  type  turbine 247 

Combustion  and  furnaces;  steam  boilers 308-311 

Combustion 277-295 

Definitions — Compounds,    elements,    heat    or    calorific    value, 
atoms,  molecules,  etc .  277-279 


404  INDEX 

PAGE 

Combustion — 
Combustion  of — 

Carbon 279 

Hydrocarbons 289,  290 

Hydrogen 287-289 

Calorific  value  of 290 

Mixtures 290,  291 

Sulphur 290 

Combustion  to — 

CO 279-282 

CO, 282,  283 

CO  to  CO2 283,  284 

Conditions  determining  formation  of  CO  and  CO3 284-286 

Excess  Air  and  excess  coefficient 286 

Flue  gases  from  combustion  of  carbon 286,  287 

Rate  of,  in  boiler  furnaces 335-337 

Temperature  of  combustion 291-294 

Theoretical  temperature 292 

Commercial  fuels — solid,  liquid  and  gaseous 296,  297 

Complete  expansion  cycle 55-58,  72 

Complete  7Vchart  for  steam 68-70 

Compound  engine 149-151 

Compounding 141-158 

Combined  indicator  diagram 155-158 

Compounding 144-149 

Cylinder  ratios 151-153 

Gain  by  expansion 141-144 

Indicator  diagrams  and  mean  pressures 153-155 

The  compound  engine 149-151 

Compounds — combustion 277-279 

Compression  and  exhaust — Bilgram  diagram 175-177 

Condensation,  cylinder,  methods  of  decreasing 89-92 

Condensation,  initial 81,  82 

determination  of 86-89 

Condensers  and  related  apparatus 251-276 

Advantages  of  condensing 251,  252 

Conversion  of    readings    from   inches  of  mercury  to 

Ibs.  per  square  inch 255,  256 

Cooling  towers 275,  276 

Measurement  of  vacuum 252-255 

Principle  of 256-258 

Types  of — 

Contact..  .   258-268 


INDEX  405 

PAGE 

Condensers  (continued) — 
Contact- 
Barometric 261-266 

Jet,  Parallel  flow 259,  260 

Siphon 266 

Westinghouse — Leblanc 267,  268 

Non-contact 268-271 

Surface 268-270 

Two-pass  or  double  flow 270-271 

Relative  advantages 274,  275 

Water  required  by  contact  condensers 271-273 

Water  required  by  non-contact  condensers 273,  274 

Condensing,  advantages  of 251,  252 

Condensing  plants 23 

Conditions  determining  formation  of  CO  and  CO2 284-286 

Connecting  rod 109,  110 

Angularity  of 179 

Conservation  of  Energy,  law  of 2 

Conservation  of  Matter,  law  of 1 

Constant-quality  lines  on  T</>-chart 66,  67 

Constant  volume  lines,  on  TV-chart 68 

Constant  speed  governing 215,  216 

Contact  condensers 258-268 

Continental  type  boiler 346-348 

Conventional  diagram  and  card  factors 125-128 

Conversion  of  barometric  readings,  from  inchs  mercury  to  pounds 

per  square  inch 255,  256 

Cooling  towers 275,  276 

Corliss  and  other  high-efficiency  engines 196-212 

Locomobile  type 210-212 

Non-detaching  Corliss  gears 201-205 

Poppet  valves 205-208 

Trip-cut-off  Corliss 196-201 

Unaflow  engine 208-210 

Corliss  engine,  trip-cut-off 196-201 

Corliss  gears,  non-detaching 201-205 

Crank  end  of  engines 98 

Cross-head  and  guides 107,  108 

Cushion  steam  and  cylinder  feed 85,  86 

Cut-off  governing 215 

Cut-off  ratio 128 

Cycle,  area  on  7>-chart  representative  of  work 73 

Complete  expansion 55-58,  72 


406  .   INDEX 

PAGE 

Cycle,  incomplete  expansion 58-60,  74,  75 

Modifications  for  wet  and  superheated  steam 73,  74 

Of  events  in  simple  steam  power  plant 22 

Theoretical,  of  steam  turbine 225-228 

Cycles,  desirability  of  various,  in  engines 55 

Cylinder,  action  of  steam  in 24 

Condensation,  methods  of  decreasing 89-92 

Efficiency 139 

Feed  and  cushion  steam 85,  86 

Ratios 151-153 

Cylinder  and  steam  chest 101,  102 

Decreasing  cylinder  condensation 89-92 

De  Laval  impulse  turbine 236-238 

Density,  specific,  of  dry  saturated  steam 38 

Description  and  method  of  operation  of  D-slide  valve 159-165 

Design  of  nozzle,  steam  turbine 228-234 

Determination  of  clearance  volume  from  diagram 131,  132 

Determination  of  I.h.p 120-124 

Developed  horse-power 137 

Developed  thermal  efficiency 138 

D-slide  valve 159-195 

Angle  of  advance 167 

Angularity  of  connecting  rod 179 

Bilgram  diagram • 169 

Description  and  method  of  operation 159-165 

Diagram  for  both  cylinder  ends 177 

Exhaust  and  compression 175 

Exhaust  lap 168 

Indicator  diagram  from  Bilgram  diagram 180 

Lead 166,  167 

Limitations  of  D-slide  valve 183-185 

Piston  positions 177 

Reversing  engines 185-187 

Steam  lap— outside  lap 165,  166 

Valve  setting 187-195 

D-slide  valve  engine,  simple 96-98 

Diagram,  Bilgram,  for  both  cylinder  ends 177 

Bilgram,  indicator  diagram  from 180-183 

Indicator 24 

Indicator  and  mean  pressures  for  compound  engines .    153-155 

combined 155-158 

Indicator,  conventional  and  card  factors 125-128 


INDEX  407 

PAGE 

Diagram,  water  rate 86,  132-136 

Diagrams  from  real  engine ; .  .  .    192,  193 

Double  acting  engines 55 

Double-flow  condenser 270 

Downdraft  furnace 315 

Draft  apparatus 370-373 

Chimneys  or  stacks 370-372 

Mechanical  draft 372,  373 

Dry-air  pump 264 

Dry-saturated  steam,  total  heat  of 33 

Specific  density  of 38 

Specific  volume  of 36-38 

Dry-vacuum  pump 264 

Dulong's  formula — combustion 301,  302 

Duplex  steam  pump 383,  384 

Eccentric 160-165 

Economy  of  turbines 247-249 

Effective  pressure,  mean,  methods  of  varying 215 

Efficiency 52,  53 

Cylinder ,. ./  . . .' 139 

Developed  thermal 138 

Effect  of  temperature  range  on 75 

Indicated  thermal 138 

Mechanical  and  thermal 137-140 

Of  boilers 363,  364 

Relative ' 139 

Elements — combustion 277 

Energy — 

Conservation  of  energy,  law  of 2 

Heat 2 

Mechanical 2 

Units  of 3 

Engine — 

Application  of  theory  for  an  ideal  to  a  real 54,  55 

Compound,  triple,  quadruple,  quintuple 148 

Receiver  type 149 

Tandem  and  cross-compound 151 

Woolf  type 149 

Desirability  of  various  cycles 55 

Double  acting 55 

Efficiency 52,  53 

Heat  quantities  involved 50-52 


408  INDEX 

PAGE 

Engine  (continued) — 

Ideal  steam 43-60 

Operation  of 45,  42 

Operation  of  the  real  steam 77-80 

Reversing 185-187 

Steam — 

Classification — 

1)  On  basis  of  rotative  speed;  (2)  Ratio  of 
stroke  to  diameter;  (3)  Valve  gear;  (4) 
Position  of  longitudinal  axis;  (5)  Num- 
ber of  cylinders;  (6)  Cylinder  arrange- 
ment; (7)  Use 92,  93 

Clearance,  volumetric  and  mechanical 84,  85 

Crosshead  and  guides 107,  108 

Cushion  steam  and  cylinder  feed 85,  86 

Diagram  water  rate 86 

Cylinder  and  steam  chest 101,  102 

Determination  of  initial  condensation 86-89 

Initial  condensation 81 

Losses,  in  real  installations 80-84 

Methods  of  decreasing  cylinder  condensation. ..  89-92 

Nomenclature 98 

Principal  parts ..........   98-114 

Bearings Ill,  112 

Connecting  rod 109,  110 

Crosshead  and  guides 107,  108 

Cylinder  and  steam  chest 101,  102 

Flywheels 112,  113 

Frame 99,  100 

Piston 102-106 

Piston  rod  and  tail  rod 106,  107 

Shaft ..    110,  111 

Re-evaporation  in 83,  84 

Rotative  and  piston  speed 93-96 

Simple  D-slide  valve 96-98 

Throttling  or  wire-drawing 82,  83 

Work  done  by 46-60 

Engines,  Corliss  and  other  high  efficiency 185-187 

Locomobile  type 210-212 

Non-detaching  Corliss  gears 201-205 

Poppet  valves 205-208 

Trip-cut-off  Corliss 196-201 

Unaflow. .  ,  208-210 


INDEX  409 

PAGE 

Entropy  diagram 61-71 

of  liquid,  vaporization,  and  dry  saturated  steam 61-63 

TV-chart  for  steam 62-65 

Complete 68-70 

Constant  quality  lines 66,  67 

Diagram  for  a  real  engine 136 

Heat  from 68 

Quality  from 65-68 

Saturation  curve 63 

Superheating  lines 63,  64 

Volume  from 68 

Water  line 63 

Entropy,  diagrams  of  steam  cycles 72-76 

Equivalent  evaporation,  boilers 361 

Excess  air — combustion 286 

Advantages  and  disadvantages 311 

Excess  coefficient 286 

Exhaust  and  compression — Bilgram  diagram 175-177 

Exhaust  lap 166,  168 

Exhaust  steam,  utilization  of,  for  heating  buildings 376,  377 

Expansion,  adiabatic 58 

Cycle,  the  complete 55-58,  72 

the  incomplete 58-60 

Gain  by,  in  compounding 151-144 

Ratio  of,  apparent  and  real 128-130 

External  latent  heat  of  vaporization 31 

Externally  fired,  return  tubular  boiler 349-352 

Fahrenheit  scale 11,  12 

Feed-water  heating 377 

Open  and  closed  heaters 377-380 

Firing  boilers  by  hand 311-315 

Fixed  carbon  in  coal 300 

Flywheel 97,  98,  112,  113 

Regulation 213,  214 

Flue  gases  from  combustion  of  carbon 286,  287 

Foot-pound,  definition 3 

Forward  stroke  of  engines 98 

Foster  superheater .  .  369,  370 

Frames  of  engines 99,  100 

Front  end  of  engines 98 

Fuel  calorimeter 302 

Fuels...  .  296-304 


410  INDEX 

PAGE 

Fuels  (continued) — 
Commercial — 

Solid,  liquid,  gaseous 296,  297 

Coal 297-299 

Analyses 299-301 

Calorific  value  of — 

Dulong's  formula 301,  302 

Fuel  Calorimeter ' 302 

Petroleum 302 

Baume  scale  to  express  gravity 303 

Calorific  values 303,  304 

Purchase  of  coal  on  analysis 302 

Functions  of  boiler  parts 306-308 

Furnaces — and  combustion 308-311 

— Updraft  and  downdraft 315 

Gases,  and  vapors,  steam 27 

— Flue,  from  combustion  of  carbon 286,  287 

Gaseous  fuels 296,  297 

Gauge  pressure 39-41 

Gearing  and  staging — turbines 238-243 

Gears,  Corliss,  non-detaching. 201-205 

Generation  of  steam  in  real  steam  boiler 38,  39 

Generation  of  steam  or  water  vapor 28 

Governing — throttle  and  cut-off 215 

Coefficient  of  regulation 216 

Constant  speed 215,  216 

Governor 97,  98 

Regulation 214,  215 

Governors — 

Pendulum 217 

Rites  inertia 218-220 

Shaft 217,218 

Grates,  mechanical 315,  316 

Gridiron  valve 185 

Guides  and  crosshead 107,  108 

Hand  firing — steam  boilers 311-315 

Heat 9 

Absorption,  reversal  of  process 38 

Energy 2 

Unit  of 13 

From  7>-ehart 68 


tNDEX  m  411 

PAGE 

Heat  (continued) — 

Latent,  of  vaporization 30,  32 

Internal  and  external 30,  31 

Of  liquid,  q  or  h 31,  32 

Of  superheat 34,  35 

Quantities  in  rectangular  cycle 50-52 

Quantity  of 16 

Specific 14 

Total,  of  dry  saturated  steam 33 

Of  superheated  steam 36 

Of  wet  steam 33,  34 

Value  of  elements  and  compounds 277-279 

Heat,  waste — in  steam  plant 375-381 

Feed-water  heating 377 

Open  and  closed  heaters 377-380 

Utilization  of  exhaust  for  heating  buildings 376,  377 

Heaters,  feed-water,  open  and  closed 377-380 

Heine  superheater 367,  368 

Heine  water-tube  boiler 355,  356 

Horizontal,  return,  tubular  boiler 306-308 

Horse-power 17 

Developed 137 

Hour,  definition 18 

Of  steam  boilers 359 

Hydraulic  analogy ; 26 

Hydrocarbons,  combustion  of 289,  290,  310 

Calorific  value  of . . 290 

Hydrogen,  combustion  of 287-289 

I.h.p. — determination  of 120-124 

Impulse  steam  turbine .   221-225 

De  Laval  type 236-238 

Inclined  stokers 322-325 

Incomplete  expansion  cycle 58-60,  74,  75 

Indicated  thermal  efficiency 138 

Indicator 115 

Indicator  diagram 24,  115-140 

Atmospheric  line 119 

Conventional  and  card  factors 125-129 

Cut-off  ratio 128 

Determination  of  clearance  volume  from  diagram.  131,  132 

Determination  of  I.h.p 120-124 

Diagram  factor  or  card  factor 126-129 


412  INDEX 

PAGE 

Indicator  diagram  (continued) — 

From  Bilgram  diagram 180-183 

Mean  effective  pressure 122 

Ratio  of  expansion 128-131 

Reducing  mechanism 118 

Scale  of  spring 118 

The  planimeter 123 

Indicator  diagrams  and  mean  pressures  for  compound  engines  153-155 

Combined 155-158 

Indicator  diagrams  from  real  engine 192,  193 

Injector,  steam 384-386 

Inertia  governor,  Rites 218-220 

Initial  condensation 81,  82 

Determination  of 86-89 

Inside  lap,  negative 168 

Internal  latent  heat  of  vaporization 30 

Internally  fired,  tubular  boilers 342-345 

Jet  condensers 259,  260 

Joule's  equivalent 14 

Joule,  the 3 

Kinetic  mechanical  energy 8 

Lap  angle 166 

Lap,  steam 165,  166 

Negative  inside .- 168 

Outside  and  exhaust 166,  168 

Latent  heat  of  vaporization 30,  32 

Internal  and  external 30,  31 

Lead 166,  167 

Leblanc — Westinghouse  condenser 267,  268 

Liquid  fuels 296,  297 

Liquid,  heat  of,  q  or  h 31,  32 

Entropy  of 61 

Limitations  of  D-slide  valve 183-185 

Balanced  slide  valves 184 

Gridiron  valve 185 

Piston  valve 184 

Riding  cut-off  valves 185 

Locomobile  type  of  high  efficiency  engines 210-212 

Locomotive  type  boiler 346 

Low-pressure  or  exhaust  steam  turbines 249 


INDEX  413 

PAGE 

Matter 1 

Law  of  conservation  of  matter 1 

Units  of  matter 3 

Mean  effective  pressure 122 

Methods  of  varying 215 

Mean  pressures  and  indicator  diagrams  for  compound  engines .    153-155 

Measurement  of  temperature 10 

Measurement  of  vacuums 252-255 

Mechanical  and  thermal  efficiencies 137-140 

Mechanical  clearance,  steam  engine 84,  85 

Mechanical  draft 372,  373 

Mechanical  energy : 2,  3,  7 

Potential  and  kinetic 7,  8 

Mechanical  grates 315,  316 

Mechanical  stokers 317-335 

Mercury  readings,  conversion  to  pounds  per  square  inch 255,  256 

Mercury  thermometers 10-12 

Method  of  operation  and  description  of  D-slide  valve 159-165 

Mixtures,  combustion  of 290,  291 

Moisture  in  coal 299 

Molecular  activity 9 

Molecules * 278 

Natural  draft,  chimneys 370 

Negative  inside  lap 168 

Non-condensing  plants 23 

Non-contact  condensers 21 3-271 

Surface  (Wheeler) 268-270 

Non-detaching  Corliss  gears 201-205 

Nozzle  design,  steam  turbine 228-234 

Oil  firing 333-335 

Open  and  closed  feed-water  heaters .   377-380 

Operation  of  simplified  steam  engine 45,  46 

Operation  of  real  steam  engine 77-80 

Outside  steam  lap 166 

Outstroke  of  engine 98 

Parallel-flow  condenser 259-261 

Parson's  type  turbine .* 246 

Pendulum  governors 217 

Petroleum 302 

Baume*  scale  to  express  gravity  of 303 

Calorific  values. ,  303,  304 


414  INDEX 

PAGE 

Piping,  steam 390 

Piston,  engine  .  .  : 102-106 

Piston  positions  for  Bilgram  diagram 177-182 

Piston  rod  and  tail  rod 106,  107 

Piston  speeds  of  steam  engines 93-96 

Piston  valve 184 

Planimeter 123 

Plant,  steam  power 20 

Plants,  condensing,  non-condensing 23 

Poppet  valves 205-208 

Positions  of  piston  for  Bilgram  diagram 177-182 

Potential  mechanical  energy 7 

Powdered  coal  stokers 333 

Power  and  work 17 

Power,  unit  of,  horse  power 17 

Pressure,  absolute 41 

Gauge 39-41 

Mean  effective 122 

Methods  of  varying 215 

Pressures,  mean,  and  indicator  diagrams  for  compound  engines.  153-1 55 

Prevention  of  smoke 316,  317 

Prime-mover 20 

Principal  parts  of  engines 98-114 

Principle  of  condenser 256-258 

Properties  of  steam 27 

Proximate  analysis  of  coal 299 

Pump,  dry  air  or  dry  vacuum 264 

Vacuum 259 

Pumps,  boiler  feed 382-384 

Purchase  of  coal  on  analysis 302 

Quality  from  7>-chart 65-68 

Constant,  lines 66,  67 

Quantity  of  heat 16 

Rate,  diagram  water 86,  132-136 

Rate  of  combustion  in  boiler  furnaces 335-337 

Rating  of  steam  boiler 358-362 

Ratio,  cut-off 128 

Ratio  of  expansion — apparent  and  real 128-130 

Ratios,  cylinder 151-153 

Reaction  type  turbine 243-247 

Receiver  engine 149 


INDEX  415 

PAGE 

Recovery  of  waste  heat 375-381 

Reducing  mechanism 118 

Re-evaporation 83 

Regulation 213-220 

Coefficient  of  governor 216 

Constant  speed  governing 215,  218 

Governors — 

Pendulum 217 

Rites  inertia. . 218-220 

Shaft 217,  218 

Kinds — flywheel  and  governor 213-215 

Methods    of    varying    mean    effective    pressure — Throt- 
tling and  cut-off 215 

Relative  advantages  of  contact  and  noo-contact  condensers.  .   274,  275 

Relative  efficiency 139 

Return  tubular  boilers,  horizontal 306-308,  349-352 

Reversal  of  process  of  heat  absorption 38 

Reversing  engines 185-187 

Riding  cut-off  valve 185 

Rites  inertia  governor 218-220 

Rotative  speeds  of  steam  engines 93-96 

Safety  and  strength  of  boilers 337-341 

Saturated  steam,  dry,  specific  volume  of 36-38 

Saturated  vapor 31 

Saturation  curve,  temperature  entropy  chart  for  steam 63 

for  compound  engine  cards 156 

Scale 365,  366 

Prevention  of 366 

Scale  of  spring,  indicator 118 

Scotch  marine  type  boiler 348,  349 

Separately  fired  superheaters 366,  367 

Separators 386-388 

Setting,  valve ...... , . . . 7,. . . , , , , , , , 187-195 

Shaft  governors 217,  218 

Shaft  of  engine. '. 110,  111 

Simple  D-slide  valve  engine 96-98 

Siphon  condensers 266 

Slide  valves 184 

Balanced 184 

Gridiron  valve 185 

Piston  valve 184 

Riding  cut-off  valve 185 


416  INDEX 

PAGE 

Smoke  and  its  prevention 316,  317 

Solid  fuels 296,  297 

Soot  and  scale,  effects  of,  in  boilers 364,  365 

Specific  density  of  dry,  saturated  steam 38 

Specific  heat 14 

Specific  volume  of  dry  saturated  steam 36-38 

Speeds,  rotative  and  piston,  of  steam  engines 93-96 

Spring,  scale  of,  indicator 118 

Stacks  or  chimneys 370-372 

Staging  and  gearing,  steam  turbines 238-243 

Steam,  action  in  cylinder : 24 

Action  of,  on  impulse  blades  of  turbine 234-236 

Boiler,  generation  of  steam  in 38,  39 

Cushion,  and  cylinder  feed 85,  86 

Diagram  water  rate 86 

Cycles,  T$-diagrams  of, 72-76 

Steam  engine  the  ideal 43-60 

Bearings Ill,  112 

Classification 92,  93 

Connecting  rod 109,  110 

Crosshead  and  guides 107,  108 

Cylinder  and  steam  chest 101,  102 

Determination  of  initial  condensation 86-89 

Flywheel  and  governor 97,  98 

Flywheels 112,  113 

Frame 99,  100 

Losses  in  real  installations 80-84 

The  real 77-114 

Initial  condensation 81 

Re-evaporation 83,  84 

Throttling 82 

Wire-drawing 82,  83 

Methods  of  decreasing  cylinder  condensation ....   89-92 

Nomenclature  of 98 

Operation  of, 77-80 

Piston 102-106 

Piston  rod  and  tail  rod 106,  107 

Principal  parts. 98-114 

Rotative  and  piston  speeds 93-96 

Simple  D-slide  valve 96-98 

Steam,  entropy  of  dry  saturated 61,  62 

Generation  of 28-39 

Heat  of  superheat 34-35 


INDEX  417 

PAGE 

Steam,  lap,  D-slide  valve 165,  166 

Modification  of  T$-chart  for  wet  and  superheated 73,  74 

Properties  of 27 

Specific  density  of  dry  saturated 38 

Specific  volume  of  dry  saturated 36-38 

Temperature-entropy  chart  for 62-71 

7>-chart  complete 68-70 

Total  heat  of  dry  saturated 33 

Total  heat  of  wet 33,  34 

Vapors  and  gases 27 

Wet,  effect  of 53 

Steam  injector i 384 

Steam  piping 390 

Steam  power  plant 20-22 

Steam  trap 388 

Steam  turbine  (see  Turbine) 

Stephenson  link  gear 186 

Sterling  water-tube  boiler 356-358 

Stokers,  mechanical 317-313 

Chain  grate 318 

Inclined,  overfeed 323-327 

Powdered  coal 333 

Sprinkler 318 

Underfeed 327-333 

Strength  and  safety  of  boilers 337-341 

Sulphur,  combustion  of 290 

Sulphur  in  coal 301 

Superheat,  heat  of 34,  35 

total  heat  of 36 

Superheaters — 

Built  in 366,  367 

Separately  fired 366,  367 

Babcock  and  Wilcox 367 

Foster.  .' 369,370 

Heine 367,  368 

Superheating 31 

Lines,  on  temperature-entropy  chart  for  steam .  .  63,  64 
Surface  condensers 268,  269 

Tail  rod  and  piston  rod  of  engine 106,  107 

Temperature 9 

Measurement  of 10 

Pressure  relations 29 


418  INDEX 

PAGE 

Temperature  of  combustion 291-294 

Temperature  rise 293 

Theoretical 292 

Temperature-entropy  chart  for  steam 62-71 

Complete  chart 68-70 

Heat  from 68 

Quality  from 65-68 

Volume  from 68 

T  (^-diagram  for  a  real  engine 136 

T(£-diagrams  of  steam  cycles 72-76 

Complete  expansion  cycle 72 

Area  of  cycle  representative  of  work 73 

Modifications  for  wet  and  superheated  steam 73 

Temperature  range,  effect  on  efficiency 75 

Temperatures  of  vaporization 29 

Theoretical  cycle  of  steam  turbine 225-228 

Thermal  and  mechanical  efficiency 137-140 

Developed  thermal  efficiency 138 

Indicated  thermal  efficiency 138 

Thermometers,  mercury 10-12 

Throttle  governing 215 

Throttling  or  wire-drawing 82 

Towers,  cooling 275,  276 

Traps,  steam 388,  389 

Trip-cut-off  Corliss  engine 196-201 

Triple  expansion 148 

Tubular  boiler,  horizontal  return 306-308 

Turbine,  steam 221-250 

Action  of  steam  on  impulse  blades 234-236 

Combined  type 247 

De  Laval  impulse  type 236-238 

Economy  of 247-249 

Gearing  and  staging 238-243 

Impulse .' 221-225 

Nozzle  design 228-234 

Reaction  type 243-247 

Theoretical  cycle 225-228 

Types  of  boilers 342-359 

Babcock  &  Wilcox,  water-tube 352-355 

Continental 346-348 

Externally  fired,  return  tubular 349-352 

Heine  water-tube 355,  356 

Internally  fired,  tubular 342-345 


INDEX  419 

PAGE 

Types  of  boilers  (continued) — 

Locomotive 346 

Scotch  marine 348,  349 

Sterling  water-tube .  .  .  : 356-358 

Wickes  vertical  water-tube 358,  359 

Types  of  condensers — 

Contact 258-268 

Barometric 261-266 

Jet,  parallel  flow  type 259,  260 

Siphon ' 266 

Westinghouse-Leblanc 267,  268 

Non-contact — 

Surface 268-270 

Two-pass  or  double  flow 270,  271 

Ultimate  analysis  of  coal 299-301 

Una-flow  engine 208-210 

Underfeed  stokers 325-333 

Unit  of  heat  energy 13 

Units  of  matter,  energy  and  work 3 

Updraft  furnace 315 

Utilization  of  exhaust  steam  for  heating  buildings 376,  377 

Vacuum 252-255 

Measurement  of 253,  254 

Pump 259 

Valve,  D-slide — (see  D-slide  valve) 159-195 

Setting 187-195 

Valves.  . .' 159-195,  205-208 

Balanced  slide 184 

Gridiron 185 

Piston -v 184 

Poppet k 205-208 

Riding  cut-off 185 

Vapor,  saturated 31 

Pressure-temperature  relations ;  saturated  water  vapor ....     29 

Water  or  steam,  generation  of 28 

Vaporization,  entropy  of 61 

Latent  heat  of 30,  32 

Internal  and  external 30,  31 

Temperatures  of 29 

Vapors  and  gases 27 

Volatile  matter  in  coal .  .  .  299 


420  INDEX 

PAGE 

Volume,  clearance,  determined  from  diagram 131,  132 

Constant,  line 68 

From  T0-chart 68 

Volumetric  clearance,  steam  engine 84,  85 

Waste  heat,  in  steam  plant 375,  376 

Recovery  of 375-381 

Water  line,  temperature  entropy  chart  for  steam 63 

Water  rate,  diagram,  steam  engine 86,  132-136 

Water  required  by  contact  condensers 271-273 

Water-tube  boilers 352-359 

Water  vapor  or  steam,  generation  of 28 

Water  vapor,  saturated,  pressure-temperature  relations 29 

Weight  of  water  required  by  non-contact  condensers 273,  274 

Westinghouse-Leblanc  condenser 267,  268 

Westinghouse-Parsons  turbine 245 

Wet  and  superheated  steam;  modifications  of  1  (£-chart  for. ...   73,  74 

Wet  steam,  total  heat  of 33,  34 

Wickes  vertical  water-tube  boiler 358,  359 

Wire-drawing  or  throttling 82 

Woolf  type  engine 149 

Work 3,  17 

Area  of  cycle  on  T$-diagram  representative  of  work 73 

Done  by  the  engine 46-50 

Unit  of..  3 


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